After you upload the avatar, refresh without cache (typically Shift+F5 or Ctrl+F5.)
Blockland Avatars have to be 75x75 pixels large.If it's an animation, it can't exceed a certain size. (Not sure what that exact size is)
Not always, as Epicduke stated. But I usually resize avatar's to 75x75 because it usually works.OP, tell us if it's a GIF or a Non-animated avatar your trying to put so we can help you.
mmmm it's 100x100 for no animated (IIRC)