Target audience is adults in 20s and 30s, with Christian backgrounds, who are educated and hip. Please don't make a cover with cheesy pastel tones like those "Christian romance" novels always have!
Ideal ebook covers have wording that is legible even when looking at the small thumbnail image, and aren't too busy, since they are viewed small. Here are other file requirements (from Kindle's site):
• Image dimensions of at least 500 by 800 pixels.
• A maximum of 2000 pixels on the longest side is preferred
• Ideal height/width ratio of 1.6
• Save at 72 dots per inch (dpi) for optimal viewing on the web
Product images display on the Amazon website using RGB (red, green, blue) color mode. RGB is the color mode native to the web and many color screen displays, as these three colors displayed at varying levels of intensity create over 16 million colors.
Use color images whenever possible and relevant. The Kindle reading device has a black and white screen today but Kindle applications for other devices, such as iPhone or PC, take advantage of color fonts and images.
Cover art with white or very light backgrounds can seem to disappear against the white background. Adding a very narrow (3-4 pixel) border in medium gray will define the boundaries of the cover.