I really think you ought to invest in a web site and do not limit yourself to just Amazon and Barnes & Noble. Your own web site should be sufficient to drive traffic, especially if you have buy links to every site which hosts your books on the book's detail page. If they are asking for .tiff or .psd, tiff is the preferred mode. Make sure that your image specs conform to the exact size of the ad, and create the tiff to include all the elements. A psd is too application-specific to be converted to any other image file type. If there is too much white space then you are not taking advantage of the whole ad space. Make your cover image bigger.
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The convention staff have accepted the ad. I want to thank you all for your help.