I’ve seen good cover art on dust jackets, pictures that let you know the artist had a halfway decent idea of what was going on inside the book.
But more often I’ve seen cover art that, to put it baldly, sucks.
For example, whoever did, or at least passed on, the pictures that grace the covers of S.M.Stirling’s books that start with Dies the Fire, obviously had not clue as to how that characters in the book really looked.
I once heard Anne McCaffrey(RIP) say that the covers of her Pern books often were poorly thought out. I mean, a dragon rider like Lessa would not be wearing a filmy white gown, would she? McCaffrey did say that the cover of The White Dragon seemed to be fairly accurate as to the gear a rider would wear. http://en.wikipedia.org/wiki/The_White_Dragon
Do you have any examples of cover art that really bugs you?