It'll probably be harder trying to find someone who will be very details and great at an intro, background, and the avatar. You should probably hold separate contests for each of the qualities. Although these qualities all deal with GFX, some people are better at photoshop than cinema 4d, and others are better at small avatars with great detail than a whole entire background.
Recommended splitting the prize into 3 contest, one for background, one for intro, and one for avatar. Although this is my opinion, just trying to help, if you have any other ideas, it's your thread go ahead and express them.