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.

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 ICO file above and choose afterwards.