What is AVIF?

Built on the AV1 video codec, and the smallest of the widely-supported image formats by a clear margin - frequently half the size of a JPG that looks the same. It handles transparency and HDR. Being new is its only real drawback: plenty of software still won't open one.

What is ICO?

Windows icons, and a container rather than an image - one file holds the same icon at several sizes, so Windows can pick the right one for a title bar or a desktop. It tops out at 256x256 and supports transparency, which is what stops an icon looking like a sticker.