This is a good read on the subject of fair use of online images in general. Obviously the question of message board avatars is so trivial neither the law nor most discussions about it deal with them specifically. It is comparable to a child wearing a Halloween mask they made by printing an image from the internet and cutting eye holes in it.
An important distinction is also that avatars are usually nothing more than a link to the ‘original work’ and not themselves an attempt to reproduce or take credit for that work.