Do not feel that you need an 'extra special' cover as a candidate for display, you would be surprised how each cover has found to be of special meaning to certain visitors. To assure consistency with the rest of the collection, the following guidelines are offered. Bigger is better. Set your scanner to a minimum of 72 DPI . If possible, set it up to 150 DPI, but this will make a larger file to send.
Bigger scans allow a much better quality result.
Don't be afraid to include a little extra around the edges of the scan, to help with registration and some other problems. Avoid processing the image with any 'draw' or 'paint' program. Just send it right as it comes from the scanner. It's less work for both you and me. It will make the file somewhat compact while maintaining color data.