We recommend that no part of your cover design ever start (or convert) to something below 300 dpi. If you’ve ever seen soft focus covers, or ones that looked like the were FAXed, it is generally because of low dpi in the layout process.

  • If you want to use a low resolution graphic, don’t enlarge it at all, and be sue you are happy with a print out of the cover before submitting it. Some extremely large files (14×18″, etc.) in low resolution, make very nice small format cover additions (4×8″, etc.).

This section is drawn from http://www.gregathcompany.com/hb/color/coverdesign.html