AVIF is a still image squeezed with the AV1 video codec, and it compresses harder than anything else in wide use - frequently half the size of a JPEG that looks the same. It handles transparency and high dynamic range. Support is good in browsers and patchy everywhere else, which is usually why people convert out of it.
Every conversion below runs in this page, on your own machine. Nothing is uploaded, there is no queue, and there is no account to make. Pick a target format, or just drop an AVIF file above and choose afterwards.