B-man thought you meant Forum Avatars. (EDIT: Oops, I misread your post - I see you got that)

If you're trying to set your IM avatar on the N900, then just open up your personal contact card in Contacts (from the menu), and tap the photo. That should set it globally, IIRC. I think you might also be able to set it per-network in the IM settings by tapping on your avatar (say, in a chat), but I may be completely wrong with that.
EDIT2: yes, both of those methods work, although I'm not sure how it behaves with multiple networks. My intuitive guess is that the photo set through Contacts->My information would override everything else, but that's just how I would have done it - I haven't tried. Also, I assume that the IM avatar functionality would be dependent on the network having support for avatars (e.g. IRC doesn't) so I'm not sure how that would affect it, either. Hmm...