[GeoffR]

There are usually 3 or 4 cover images per book, depending on firmware version. All of these are removed when you remove the book.

However there are also cover images associated with recommendations, and there is no option on the device to remove recommendations. But you can remove them using Calibre: Tick the "show recommendations" box in the KoboTouch driver configuration, and then use Calibre's remove book function.

Edit: There might also be other images in the .kobo-images directory from books that have the "Beyond the Book" feature enabled, and from awards, related books, wishlist books, etc.