Convert

JPEG suits photographs, PNG keeps sharp edges and transparency, and WebP is usually the smallest of the three. AVIF is used only when the browser can encode it.

About this tool

Save a picture in another format so it fits where it is going. JPEG keeps photographs small, PNG keeps sharp edges and transparency, WebP often beats both, and AVIF is offered when the browser can actually write it.

Questions

Why is AVIF marked as not supported?
Encoding AVIF is left to the browser, and some browsers can decode it without being able to write it. The page encodes one pixel and checks what format comes back before offering the choice.
What happens to transparency in a JPEG?
JPEG has no alpha channel, so a transparent area has to become something. A white layer is laid down first, which is the usual choice for a photo going to a place that expects a solid background.
Which format should I pick?
Photographs are usually smallest as WebP or JPEG. Screenshots, diagrams and anything with text or a transparent background hold up better as PNG, and the size shown for each choice makes the trade visible.