What version of vBulletin are you using. If your using vBulletin 3.0 series there is a simple hack over at vBulletin that I use on my site to give them a default "No Avatar" seclected in all there posts as a member if they don't choose one. It's exactly the hack your looking for, as it also adds a link to the avatar that when clicked takes them to change there avatar in there profile as well.
What version of vBulletin are you using. If your using vBulletin 3.0 series there is a simple hack over at vBulletin that I use on my site to give them a default "No Avatar" seclected in all there posts as a member if they don't choose one. It's exactly the hack your looking for, as it also adds a link to the avatar that when clicked takes them to change there avatar in there profile as well.
Go here to check out there other simple hacks, there are quite a few nice ones to install that are very very useful. Like the eMail Validation warning hack.