Here are just some small rules for the artwork:
~Icons should be SQUARE. I find that 150x150 works best and gives a better resolution but of course any size works as long as they are square.
~Banners MUST be 800x100 or a larger equivalent. Anything smaller will be pixely and anything larger has the potential to cut off your work.
~Backgrounds should be high definition wallpaper size. Fanpop states they should be at least 1280 pixels wide. Anything larger is fine.