As a prospective seller, I have to say that it is a little off-putting (word?) to look through the list of sellers (I do this sort of thing) and see all the blanks instead of avatars. It makes the place seem a little deserted. I've been aware of the site for a long time though, and appreciate the form and would like it to be a top choice.
Maybe a general email could even go out to sellers who need avatars and banners? I think it may be the right approach to offer help - many may have no idea how. Having a couple of people who offer graphics as a business would really improve participation, too, I think.
There's at least one here who offers avatars and banners, and she's terrific at it.
I've often been tempted to private message sellers through their shops and tell them, "Look, you're not going anywhere without an avatar, banner, shop description, item descriptions, and non-blurry, non-yellowed, awful photos!" That doesn't even take into account the misspellings, which both look unprofessional and don't help you get found in searches for an item!
But then I think they'll get defensive, not listen, and I'll just be known as a know-it-all b**tch. Which I probably am, but at least I'd like to be a helpful know-it-all b**tch. Y'know?