I mostly agree with what has already been said. If you look at the Featured Games their cover images generally include their title and an illustration in a style that tries to evoke the feel of the game. However it is hard to know if that format of cover image has contributed to those games being succesful or if it is just something that people have copied after seeing other Featured Games use it. Like DarkBloodbane, if a cover image catches my eye I mouse over it and if there are no screenshots or gameplay video I move on. It surprises me the number of games that do not have any screenshots or video, even sometimes when they are expecting people to pay up front!
I would add that in general people are drawn to human faces and characters, so if you can work them into your cover image it is going to be more eye catching than one without.