JPG vs JPEG: Image Formats Explained

There’s a great website called Fileinfo.com. It is a one-stop reference point for common and not-so-common file extensions. Every file format from JPG to TIF and D64 (the last named format being the Commodore 64’s disk file format on the slow 1541 disk drive). Inspired by this website is our look at the file formats you come across. For the first article in this series, we look at the JPG file format. In website design, JPEG is one option alongside PNG, WebP and AVIF.

The JPEG standard was created in 1992. Around that period, we were dabbling with Amigas; some of us at school used BBC Master System and Acorn Archimedes computers. At home, Commodore 64s, ZX Spectrums, and Amstrad CPCs were popular with younger children (whose elder siblings bought Amigas and Atari STs).

Landscape image crop and photographs in different aspect ratios prepared for a website.
Concept illustration: landscape image crop and photographs in different aspect ratios prepared for a website.

JPG vs JPEG: two extensions, one format

The .JPG and .JPEG extensions refer to the same image format. JPEG stands for Joint Photographic Experts Group. It is widely used for photographic images on websites. Common web JPEGs use 24-bit colour, which describes colour depth rather than pixel dimensions. Their usual lossy compression reduces file size at the expense of some image detail.

Lossy compression discards some image information to make a smaller file; stronger compression can leave visible artefacts. Enlarging an image is a separate issue. For example, a 640 × 480-pixel image enlarged to 1280 × 960 does not gain genuine detail. At 2560 × 1920, softness or visible pixels may become more obvious. Keep a high-quality original and export appropriately sized copies for the website. Repeated editing and lossy re-saving can introduce further quality loss.

JPEG remains useful for photographs, but the best choice depends on the image. Compare file size, quality and features using MDN’s image-format guide. For practical preparation advice, see how to get more from your images for website design.

Dreamkatcha, 20 June 2017.

JPG graphic image by Jane Kelly (via Shutterstock).

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