About JPG and AVIF
JPG is the long-standing lossy photo format. It compresses smooth gradients and photographic detail well, but it has no transparency and shows artifacts around sharp edges at lower quality settings.
AVIF is the newest of the four, built on the AV1 codec. It usually produces the smallest files, handles transparency and wide color, and is supported by all modern browsers — encoding just takes longer.
Converting JPG to AVIF is lossy — use the quality slider to trade file size against fidelity. Files are converted on the fly and never stored.