The splash screen is not as flexible as you think. Anyway, part of the reason for these contests is to bring people to the forums. As to the post limit, ifthey're not assed to make a minimum of ten posts, then the point of getting them here to the forums is somewhat lost, plus it cuts down on vote tampering. Now, even assuming we could setup a random numbering sequence on these forums, it doesn't stop vote tampering, as a person could simply provide a link to his graphic in-game or indicate a word or identifying marker, so people know what to vote for. If you're going to argue the first-gets-most argument, these particular polls don't seem to have that flaw, particularly since it's not forced on people and, instead, requires they go out of their way to participate.
Hope that answered most of your questions.