Which format, in one paragraph each
JPG is for photographs. It discards detail to get small, which suits continuous tone and ruins hard edges. It cannot store transparency; anything transparent becomes white.
PNG is lossless. Every pixel survives exactly, transparency is supported, and files are large for photographic content — often ten times a JPG of the same picture. Right for screenshots, logos, diagrams and anything with text.
WebP does both jobs: lossy like JPG but smaller at the same quality, with transparency. Supported everywhere that matters now. It is the sensible default for the web if you do not need to hand the file to someone who will open it in older desktop software.
AVIF is smaller again, often dramatically, and slower to encode. Browser support for viewing is broad; support for writing is not, which is why it may not appear in the list here.
Why the list changes between browsers
This tool asks the browser to encode a two-pixel test image in each format and checks what actually comes back. Safari, for instance, cannot write AVIF, so on Safari AVIF is absent from the menu rather than present and silently producing a PNG with the wrong file extension.
That failure mode is more common than it should be, and it is nasty: you get a file that looks converted, is named as though it were converted, and is not. Testing capability rather than sniffing the browser avoids it entirely.
What conversion cannot undo
Converting a JPG to a PNG does not restore the detail JPEG discarded. The result is a lossless copy of a lossy image — larger, and no better. Going the other way loses more each time, so a picture converted back and forth repeatedly degrades visibly. Convert from the best original you have, once.
Transparency is lost the moment you convert to JPG, replaced with white. If a logo needs a transparent background, PNG and WebP are the only options here.
Everything here happens in this browser tab. The image is decoded, redrawn and re-encoded by the Canvas API on your own device, and the result is assembled in memory and handed to you as a download. Nothing is uploaded, which you can confirm in about fifteen seconds with the network panel open.