If we are talking about banners (as I did post on GPWA in response to Satyas question) I have seen that the best converting creatives, banners or splash pages alike, are the ones that are targetted..
For example, if you are targetting players who want to play progressive jackpots, the best banners are going to be ones that have references to the jackpots, pictures from the games, and lead to a splash page that has more information about that game..
I had some banners made a while back and some of them have the lion from mega moolah on them. The colours on the banner just make it stand out and the lion is awesome..
Anyway, this banner has received a LOT of attention and the guys I have recommended it to have given me positive feedback…
Images are all about the target audience. You’re never going to please everyone, but you should try to stick with something that is relevant to the traffic you are trying to attract..
And as Dom said, the banners need to go with your site too.. You cant just stick a fluoro green banner on a red background… not if you want them to stay…