They have very specific ideas of what they want. If you look at the most popular list and check out the highest rated on the left, that seems to be the kind of thing they are after. I find it really odd because if you search for illustrations the and most popular none of this kind of illustrations appear in the list.
I don't know if they just want you to prove that you can draw before they let you upload more stock related images or they have just tightened up what they are accepting.
Or maybe I am not the vector god that I thought I was! LOL