Insane Difficulty: User Pic/Forum Avatar Requirements? - Insane Difficulty

Sorry if this has been asked before, but I absolutely, positively did not see this listed anywhere in any of the parts of the forums that I'd visited. Doesn't mean it isn't there -- just means I didn't see it.
That said... what are the requirements to earn a user pic/forum avatar? X amount of posts? Y amount of time on the forum? X amount of posts in Y amount of time? Or is it as simple as politely asking the right people/person?

Thanks in advance.