Now you know the psychology behind book sales you need to know who your market is
. What appeals to them? If you are selling a "how to make money" ebook then you might want to include ebook graphics that depict the trapping of financial success. If you are appealing to a weight loss market then appeal to the sexy, slim, stereotype that people strive to become. If it is fishing, you get the picture, the biggest fish you can find (or just use a little fish and some creative Photoshop editing). You see one size doesn't fit all. Know your market. If you are not sure what appeals to your market all you have to do is look at the big players in that market. They are doing something right aren't they? Just analyze their graphic design and choose similar photos or graphics. They've already done all the hard work for you! Obviously I'm not advocating plagiarism just "market research".