Any book, particularly an eBook, stands or falls on the strength of its cover image. That's a given. There's even a well-known phrase that goes with that sentiment. Any potential reader would not look once, never mind twice, at the contents of your book if the cover was not a work of art. Also, covers must have a connection with that story in some way. There is no point in having a sci-fi tale full of political intrigue, or interplanetary warfare, or even simple space travel and have the cover showing a child playing. Unless, of course, the child was central to the tale.