Time was, there was something called vanity press. If you wanted your book published as an actual paperback, you contacted a vanity press, went through the agony of involved preparations, galley sheets and all the expenses that went with it. Then came the best part:
You had to order quantities of books at a time,pay for them and hope you could sell them.
Today there is P-O-D, or Print-On-Demand. You can publish a book, not purchase any for yourself, and when an order comes in, they can print one book at a time. Thus, Print-On-Demand.
P-O-D Covers Require A Bit More Information
Looking at the above example better explains the P-O-D difference. The book has a front cover, a back cover and a spine. You will have an assortment of book sizes to choose from, which determines the overall size of the cover. Plus, as said, your book has a spine. The spine width can only be determined based on the number of pages times a numerical formula determined by the dimensions of the book. All graphics must be high resolution (that's our job). But if you have a favorite graphic you want incorporated in your cover, it must be 300 dpi at the size we need it. If we have to enlarge your image, it will lower the resolution and the graphic will be fuzzy.