As far as I can see, there's no real need for a separate gallery. An artist could put the variety in the wallpaper/dock/icon screenie and maybe add a tag... similar to dA.
It isn't always that easy. I have done it here, with certain of my wallpapers, but many were made *only* for mobiles. Anyway, if WC wanted to make a separate gallery for mobile walls, I'm sure they would have already. And besides, just like walls for pc's, anyone with half a brain can make a wallpaper for themselves.