Now I don't know how vBulletin 2 works - but at least on 3 you can automatically promote users to a different user group at attaining a certain level of posting.
Could you not set up a seperate user group which is a complete copy of the registered users usergroup; but with Custom Avatars. Then when you reach a certain posting level - I don't know, 20, 50, 100? it will automatically promote you in a sense to the copied usergroup and then you can upload your avatar.
Nobody with more than 20, and certainly no more than 50 Posts is going to even think of using you as an image host - especially not at 65 by 65. No new users looking for a image host would even know about it; so it wouldn't cross their minds?
Just a suggestion though.
NOTE: I do not believe we should have vb
code sigs and especially not image sigs.