already discussed...
it's been said it's something that just won't happen...
it was tried before. and *somehow* abused so David turned it off.
I'm not sure how it was absued, but it just was.
there is also some quirk of the setup that not all mods have the ability to change avatar pictures, it's not practical to elect a chosen few mods, because all the mods are equal, (the fact that I can change your picture whilst someone else can't is irrelivant, because at the moment I can't add a custom avatar for you anyway).
besides, which...
even if we were allowing custom avatars, it'd likely be for the chosen few in the same way that the titles are now.
I guess the message of the story is, get posting and helping people and you'll likely be rewarded.