Right now the NaughtyDog.com community forums allow you to pick within a built-in set of pre-defined avatars through the "Avatars" tab on your individual "My Settings" configuration page.
You can get there by going to: http://forums.naughtydog.com/t5/user/myprofilepage/tab/user-icons
One of the updates we've got planned to the forums in the near future is to pull in your selected PSN avatar instead -- working in very similar fashion to the PlayStation.com forums (and other PSN login enabled forums).
We'll keep you updated when we get a better idea of when that funcationality will be enabled.
These were pretty sweet when I saw them and picked them for the forums here. I just feel for the sake of consistency it should be your PSN icon since that way you'll be somewhat identifiable across a bunch of PSN enabled sites and online versus custom ones only here. The downside is, of course, limited choices for avatars for the time being.