What an AVIF file actually is
AVIF wraps the royalty-free AV1 video codec as a still-image format. It compresses harder than anything else in wide use — commonly around half the size of an equivalent JPEG — and adds real range: 10- and 12-bit colour, HDR, wide gamut and an alpha channel.
AVIF files come from modern websites and image pipelines that adopted it for the bandwidth saving. Chrome shipped support in 2020 and Safari in 16.4 in 2023, so it is common on the web and rare on the desktop.
What you get from JPG
Saving as JPEG re-encodes the image with lossy DCT compression at a quality level you choose. Around 80% is the usual sweet spot: files shrink dramatically and the difference is invisible at normal viewing sizes.
Nothing in mainstream computing fails to open a JPEG. Thirty-year-old software, print shops, hospital systems, government upload forms, every phone and every browser — if a system takes images at all, it takes JPG.
What changes when you convert AVIF to JPG
Both formats are lossy, so this is a re-encode rather than a copy: the AVIF is decoded to pixels and those pixels are compressed again as JPEG. Detail the original already discarded cannot come back, and the second pass removes a little more. Going to JPEG specifically means giving up the more modern codec's efficiency — expect the file to grow, sometimes considerably, for the same apparent quality. That is the price of a format everything can open.
Transparency is the thing to watch. AVIF can store transparent areas and JPG cannot, so anything see-through is composited onto a white background during conversion. A logo with soft edges will show a white fringe when placed on a coloured background afterwards. If the transparency matters, convert to PNG or WebP instead.
One thing worth checking before you convert: AVIF can store 10- and 12-bit colour, HDR and a wide gamut, and JPG cannot. If the file came from an HDR pipeline it is tone-mapped down to 8-bit SDR here, which is usually invisible on an ordinary screen but flattens the highlight range a display capable of showing it would have used.