What is a JPEG? What it stands for, JPEG vs JPG, and how to make one smaller

The short version

• It’s the everyday format for photos. Mozilla’s guide for web developers calls JPEG “currently the most widely used lossy compression format for still images.”
• JPEG stands for Joint Photographic Experts Group, the committee that made the standard in 1992.
• JPG and JPEG are the same. We saved one picture as .jpg and as .jpeg in two different programs; each time the two files were byte-for-byte identical.
• It’s small because it throws detail away. That suits photos, not logos or screenshots, and a JPEG can’t have a transparent background.
• To make one smaller, shrink the pixels first. On our test photo, 1,200 pixels wide instead of 2,752 made the file 81% smaller at the same quality setting.
• Start from the original. Saving an existing JPEG again at a lower quality gave us a bigger file, further from the original, than saving the original at that quality.

What is a JPEG?

A JPEG is a photo saved in a format that makes the file small. Mozilla’s image file type and format guide describes JPEG as “currently the most widely used lossy compression format for still images” and adds: “It's particularly useful for photographs”.

Lossy means some detail is thrown away when the picture is saved, and it doesn’t come back. For a photograph that’s usually fine. In the guide’s words: “Photographs and other continuous-tone images can usually tolerate lossy compression, because the discarded detail is harder to see.” Sharp edges and text are where it shows, which is why the same guide warns that “applying lossy compression to content requiring sharpness, like diagrams or charts, can produce unsatisfactory results.”

The format is an international standard, ISO/IEC 10918. The JPEG committee’s page on it says it was “created in 1992 (latest version, 1994)”, after work that began in 1986, and that it is “still the most dominant still image format around”.

What does JPEG stand for?

JPEG stands for Joint Photographic Experts Group. It’s the name of the committee, not of the file. The committee’s About page introduces “The Joint Photographic Experts Group (JPEG) committee” as a working group of two international standards bodies, ISO and IEC.

The “Joint” isn’t about those two working together, though. The committee says it comes from the fact that its work is “the result of an additional collaboration with the International Telecommunication Union (ITU)”. It’s usually said “jay-peg”, as Mozilla’s guide notes.

JPEG vs JPG: is there a difference?

No. .jpg and .jpeg are two endings for the same kind of file. Mozilla’s guide lists .jpg, .jpeg, .jpe, .jif and .jfif as endings for the one format, all with the same type label, image/jpeg, which is what a web server sends to say what a file is. Google Merchant Center lists the format as “JPEG (.jpg/.jpeg)”, and Google Business Profile, which says “Format: JPG or PNG”, means the same format.

We tested it rather than take it on trust. On 5 November 2026 we took one of this blog’s share images and saved it twice from the same program, once with each ending. We did it in Pillow (the Python imaging library) and again in ImageMagick. Each time, the .jpg and the .jpeg were byte-for-byte identical: the program wrote exactly the same file, and only the name differed. Both begin with the same few bytes, which include the letters JFIF. That’s the “JPEG File Interchange Format (JFIF)”, the file layout set out in part 5 of the standard.

So renaming a .jpeg to .jpg, or the other way round, changes nothing. Why are there two endings at all? The usual story online is that MS-DOS and early versions of Windows only allowed three-letter file endings. We couldn’t find that on the JPEG committee’s pages or in Mozilla’s guide, so we’re not repeating it as fact. Either way, the file is the same.

Renaming isn’t converting

Renaming between different formats is another matter. We renamed our PNG logo to .jpg: it was still a PNG inside, transparent background and all, and both Pillow and the standard file-checking tool on Linux said so. Google’s image guidance says “It's also a good idea to have the extension of your filename match with the file type.” Google Merchant Center’s image rules are firmer: “Using incorrect file extensions (for example, .jpg for a .png file) can lead to 'Unsupported image type' errors.”

To turn a PNG or an iPhone’s HEIC photo into a JPEG, convert it (below); don’t rename it. Our post on HEIC files covers the iPhone side.

What a JPEG is good for, and what it isn’t

Mozilla’s guide puts it in one line: “JPEG is lossy, so it is often a good fallback format for photographs, while PNG is lossless, making it better for screenshots, diagrams, and line art.” In practice:

• Use JPEG for photographs: products, your premises, your team, your work, anything taken with a camera.
• Use PNG for logos, screenshots and anything with text in it. JPEG blurs sharp edges. Our post on PNG files explains why.
• Never JPEG for a transparent background. Mozilla: “JPEG does not support an alpha channel.” The alpha channel is what makes pixels see-through. When we saved our logo as a JPEG, its clear background turned black; the full test is in JPG or PNG for your website.

Newer formats can beat JPEG on size. Mozilla’s advice for websites is “For raster images, prefer WebP or AVIF, which generally provide better compression than PNG, JPEG, and GIF.” But not everywhere takes them: Google Business Profile still asks for JPG or PNG. Our post on WebP files has our own numbers.

How to open a JPEG

Almost anything opens a JPEG. Mozilla lists its browser support as “All versions of Chrome, Edge, Firefox, Opera, and Safari”, so dragging the file into a browser window works.

• On Windows: Paint opens and saves JPEGs. Microsoft’s Paint page says “you can save your work in various file formats, including PNG, JPEG, BMP and GIF, among others.”
• On a Mac: Preview opens them, and Apple’s guide to converting files in Preview lists JPEG among the formats it can save.

How to make a JPEG smaller: what we measured

Two things decide a JPEG’s size: how many pixels it has, and the quality setting it was saved at. We measured both. On 5 November 2026 we took one photograph, the picture behind this post’s share image before we cropped it: 2,752 × 1,536 pixels, 4.9 MB as a lossless PNG. We saved it as a JPEG at different settings with Pillow and measured each file. To check the smaller files weren’t simply worse, we used PSNR, a standard measure of how far a compressed picture has drifted from the original: higher is closer.

* Measured against the original resized to the same 1,200 × 670 pixels, since PSNR compares pictures of the same size. KB here means 1,000 bytes.

What the numbers say:

• Pixels matter most. At the same quality, 85, the photo at 1,200 pixels wide was 45 KB against 234 KB at full size: 81% smaller. If your website shows a photo 1,200 pixels wide, the extra pixels only add weight. Google Business Profile’s photo guidelines give “Recommended resolution: 720 px tall, 720 px wide.”
• Quality 95 is expensive. Going from 95 to 85 cut the file by 66%. Each step down saved less than the one before: 459 KB, then 83 KB, then 31 KB, while the picture drifted further from the original each time.
• Don’t keep photos as PNG. At full size and quality 85, the JPEG was 95% smaller than the PNG.
• Start from the original, not from a JPEG. We saved the 1,200-pixel photo at quality 85, then opened that JPEG and saved it again at 75. It came out at 40,885 bytes and 42.4 dB. Saving the original straight at 75 gave 34,052 bytes and 43.7 dB. So the second-hand file was about 20% bigger and further from the original.
• Re-saving at the same setting did little harm. Opening the JPEG and saving it again at quality 85, ten times in a row, moved it from 44.9 dB to 44.7 dB. In our test the real losses came from the first save and from changing the setting.

Doing it yourself

• Windows, in Paint: Microsoft says “you can resize an image in Paint by selecting the "Resize" option from the "Home" tab. You can then specify the new dimensions for your image in pixels or as a percentage of the original size.” Then save it as a JPEG.
• Mac, in Preview: Apple’s steps to reduce an image’s file size are to choose Tools > Adjust Size, select “Resample image” and enter a smaller value in the Resolution field; the new size is shown at the bottom. To change the quality, use File > Export: Apple says “If you choose JPEG or JPEG 2000, you can adjust the image’s quality.”

Whichever you use, keep the original and save the smaller copy under a new name.

Does Google accept JPEG?

Yes, everywhere a small business is likely to need it:

• Google Search: “Google Search supports images referenced in the src attribute of img in the following file formats: BMP, GIF, JPEG, PNG, WebP, SVG, and AVIF.”
• Google Merchant Center: its supported file formats start with “JPEG (.jpg/.jpeg)”.
• Google Business Profile: “Format: JPG or PNG”, and “Size: Between 10 KB and 5 MB.”

JPEG 2000 and JPEG XL aren’t JPEG

The JPEG committee has made other image standards since, and two of them turn up in menus and search suggestions: JPEG 2000 and JPEG XL. They’re different formats, not new versions of the file you already have. Preview offers JPEG-2000 as an export format. Mozilla describes JPEG XL as “standardized as ISO/IEC 18181” and says “Browser support is not yet universal.” Neither is on Google Search’s, Merchant Center’s or Business Profile’s list, so for a website or a listing, a plain JPEG is the safe choice.

What argues the other way

• Our test is one picture. It was AI-generated, dark and smooth, and pictures like that compress unusually well. A busy, grainy photo will be bigger at every setting. The direction of the results is common knowledge; the exact numbers are ours.
• Quality numbers aren’t universal. Ours are Pillow’s 0 to 100 scale. Other programs have their own controls, so judge by the file size and by eye, not by the number.
• PSNR is one measure. It counts how far the pixels moved, which doesn’t always match what people notice.
• JPEG isn’t always the smallest choice. WebP and AVIF usually beat it for photos on a website, if everywhere you need to upload to accepts them.

Sources, and what we checked

• The JPEG committee: “Overview of JPEG 1” (the standard, its dates and part 5, the JFIF file format) and “About JPEG” (the committee and the “Joint”).
• Mozilla, Image file type and format guide: what JPEG is for, its file endings, transparency, browser support, WebP and AVIF, and JPEG XL.
• Google, Image SEO best practices; Image link [image_link] (Merchant Center); Manage your Business Profile photos and videos.
• Microsoft, Draw, Create and Edit with Paint; Apple, Convert image file types using Preview on Mac and Crop, resize or rotate an image in Preview on Mac.
• All of the pages above were read in full on 5 November 2026, and again on 7 November 2026, the morning we published. Every quotation was checked word for word against the page it comes from, keeping the sources’ own spelling and punctuation.
• Our tests, run on 5 November 2026 with Pillow 12.2.0 and ImageMagick 6.9: the .jpg and .jpeg files were saved from one of this blog’s 1,200 × 630 share images at quality 85; the size and re-save tests used the 2,752 × 1,536 picture behind this post’s share image, an AI-generated photograph.