I'm thinking it might be best to add guidelines when editing the profile theme.

For example, not everyone is going to know that a favicon should be 16x16 and be uploaded as favicon.ico

Otherwise, the very idea of completely customizing the site based on your personal preferences is pretty damn neat. :)

For the favicons, isn't the site resizing them to favicon size anyway? It seems to be working fine for now without forcing people to learn how to make a proper .ico file.

Ah, well in that case there's no problem.

The favicon was just an example. There's the background, foreground, and logo - what's the limit on file size? what width and height do you recommend your upload to be? And some users might not even know what a favicon is.

This might vary for each user with their own viewports and resolutions, but guidelines for this stuff is standard design practice.

Anyway, I can't imagine it's a massive priority for the developers right now. But it's something that should be put on the list.

The site does indeed convert images to appropriate resolution/format if needed.

Some work has been done on an F.A.Q. but of course actual development takes priority.

As for sizes (subject to change, of course), favicons should be square and are resized if they are larger than 64x64. The logo should be 180x32 and is resized if larger. Back- and foregrounds do not have a hard resolution limit at this point in time, just don't go overboard as bigger image = slower page load.