Get Puff to do the graphics!?
EDIT: One huge suggestion for you, Apoc: When people submit their own maps, allow them to have 'Map Packs' - multiple "similiarly themed maps" that, when a "certain level" is reached, you have the option of continuing to play, or go to the next 'map'. And allow people to reskin towers, GUI, ect for each map pack - allowing people to recustomize your game and make a mini modding community. :O