The folks over at the Abebooks blog have published their picks for 30 Books Worth Buying For the Cover Alone. Only seven of the 30 use photographs. So illustration seems to be the way to go if you want to get noticed in the cover design crowd (of course, these are all fiction titles).
I REALLY like these two:
But have to wonder about this choice:
The image just seems to literal and obvious to be chosen as worth “buying for the cover alone”.