I don't have much to add beyond what the others have said, but I'll add another voice to the weight of the suggestions. Remove the app store logo. It's redundant and distracting. If you're going to have the cars on the banner, and there is going to be a background behind them, then make that background interesting - a twisty track seems like a good idea to me. Do some custom renders of the cars for the banner, not just resized sprites. Just my opinion, but "Innovative Unique Awesome" don't really tell me much about the gameplay and in fact lead me to question if the claims are true. Maybe this is good because I find myself thinking about the game, but if you could use terms a little less vague or "commonplace" I think it would be an improvement. You'll have more space to work with if you omit the App Store logo anyway. Another random suggestion, but maybe have the 2nd car in the image beat up and damaged. If I can see how a nice stunt car went from a polished red model to a battered piece of nigh-scrap metal, then that tells me something evocative about the game in just 2 sprites (not that you have vehicular damage, but that driving around in your game is going to be wild and dangerous).
I just read your post, and I really like the damage idea, I'll probably try that next. Here is what I have now.
I think it looks better now, but I still don't think you need two cars there. I mean we get it… the game has cars in it, but what else does the game offer that makes you want to play it?
