What is a JPEG file?

JPEG was designed for photographs, and it remains excellent at them. It exploits two facts about human vision — that we resolve brightness better than colour, and coarse shapes better than fine texture — to discard information nobody misses.

The result is a photograph at a tenth of its lossless size with no visible difference at normal viewing. Thirty years of newer formats have improved on that by 25–50%, which is meaningful but not revolutionary. JPEG got the hard part right first.

Transparency. JPEG has no alpha channel. When a transparent PNG is converted, every transparent pixel must become an actual colour, and most tools silently choose black — which is why logos exported to JPEG so often arrive inside an unwanted dark box.

Sharp edges. Describing an instantaneous jump from black to white requires an infinite series of wave patterns. JPEG keeps a finite number, and the truncated series overshoots around the edge, producing ringing — faint ghost echoes beside every letter. Chroma subsampling compounds it on coloured text.

Both are structural. No quality setting fixes either.

Every time a JPEG is decoded, edited and saved again, the encoder re-quantises data that has already been quantised. Errors from the previous pass are treated as real detail and given errors of their own.

This accumulates in one direction only. Saving an already-compressed JPEG at quality 95 does not restore anything — it just spends more bytes preserving the first pass's artefacts. The smeared, blotchy look of an image that has been through a dozen chat apps is generation loss.

The habit that avoids it: always compress from the highest-quality original you have, and do it once.

Compatibility, and it is not close. Every camera writes it, every printer accepts it, every email client displays it, and every piece of software written in the last thirty years opens it. WebP and AVIF are web formats; outside a browser their support is still patchy.

Decode speed is the other advantage. JPEG decoders are old, extremely optimised and frequently hardware-accelerated, which matters on low-powered devices displaying many images.

So: WebP or AVIF for images on a web page, JPEG for anything you send to a person, hand to a printer or attach to an email.