Book covers for ebooks? Actually an image which draws possible readers to take look at the title, the blurb, and perhaps to read at least a sample of the writing.
When deciding on mine, so far I've tried to get distinctive, even evocative images. Bright clear colours with not too much clutter.
They are viewed onscreen as thumbnails, so writing has to be easily legible.
The other aspect which occurs to me is what happens on the ereader screen? Those colours are useless on a Kindle or Kobo or whatever, because everything is black and white. These images have be distinctive in grayscale.
Few indies seem to have realised this.
Readers looking for books on Kindle or Chapters etc do so with their ereaders. They can't see those fancy colours. Ok there are people browsing with their iphones, colour Nooks etc, but let us try to have images that work on all screens.