If you find yourself in this situation where you want to produce an eBook cover (not a print book) I would make these suggestions:
• Book cover size (The online retailer will tell you the best size to create the artwork)
• Just the cover image is needed. No need for spine, back cover or flaps
• Be sure the design includes your book title AND your author name very large
• Use color and contrast to help the book cover “pop”
• Keep it simple
Above, Catfish and Mandala, is an example of a cover design I think could be better if it is only available online. Notice how the author’s name is hard to read? Also, notice the sub-text is difficult to read. I do love the graphic and the orange color!
Above right, The Bone Season, is a cover I think works really well strictly in an online environment. It has both the title and author name very visible and it enjoys some nice graphics as well.
Many authors publish both a print version and an online version of their book. If you are in this camp, your graphic designer can help you create a cover version that can work well in both scenarios. Or they can create two covers, one for each environment, keeping the look and feel consistent. Good luck!