What is ICO?
ICO is the Windows icon container, and still what a browser fetches when it asks a site for /favicon.ico. One file can hold several sizes at once so the system can pick the right one; each dimension is stored in a single byte, which caps an icon at 256 pixels.
What is TGA?
Truevision TGA is a fixture of game development and 3D texturing, where its straightforward layout and real alpha channel have kept it in use long after browsers stopped caring about it. Files may be raw or run-length encoded.