Be careful if scaling (enlarging or reducing) graphics. You can have a very high quality graphic, but if you enlarge it 800% to fit your idea of the cover, it will generally loose focus and/or pixilated. The lower the quality, the less you can enlarge a graphic. As a rule, reduction is not a quality issue, unless it is shrunk so small that little or no detail is visible. Keep in mind, if using a photo quality, or other high quality computer printer to view your graphic print outs that production machines do not always match or exceed this high end, consumer quality – unless specifically commissioned.
Graphic files that are layered should be sent in their native format, as well as being flattened into a single layer, for original artwork. If sending PDF, be sure that the conversion “locks down” all fonts.
This section is drawn from http://www.gregathcompany.com/hb/color/coverdesign.html