Your back cover is as important as the front cover.
Even if your are going to sell your book strictly online, you must have a back cover. There are several things to include on the back of your book:

1) Put a recent, high-quality photograph of yourself on the back. Not too large, but clear enough for you to be recognized. Don't edit your image from a group picture of your uncle's wedding.

2) Summarize your book for the book lover browsing for a new and exciting author to read. Don't give away the plot, but make it exciting enough for them to want more. Limit the summary to less than 200 words. Ask questions about one or more characters. "Can Julie find true love after her accident?" "Is Zake's ghost ever to find peace?"

3) If at all possible, include a review. Not by the uncle from the wedding picture, but a professional person. You might have to send a free copy of your MS to several people, but make a critical review of your work part of the give-away.

4) Purchase an ISBN. Include the ISBN and the Price on the back.

5) Include the photographer or Cover Editor's name, if you used them. They appreciate recognition, too, even if its small and at the bottom. "Cover Designed by: ", or something similar.