Here is the cover for my book.
The particulars of this cover are 1800 x 2700 pixels at 300 dpi which translates to 6 x 9 trade book size. The high resolution is needed for a clear image when the cover is printed. When used as an e-book cover a lower resolution/size will be used.
The artwork is two images licensed from http://www.istockphoto.com. Having the right to use the images is important. Either you must own them or license them. The license fee here was about $40 which allows me 499,999 impressions. After that the cost goes up, which I would welcome.
My daughter the graphic designer, who helped me with the initial cover, wanted the lettering to look "edgy." The original font looked really good, but became unreadeable when shrunk down to a thumbnail. Since e-readers and images on bookseller sites will display the tinier version, I changed it.
The actual story is not as graphic as the images display. I polled a number of people including a mother and a twelve year old and everyone seemed fine with the images. The look is more likely horror, although the story fits more in the thriller/mystery genre. Also, my daughter shadowed the knife to look like it was lit by the candles, which is good since I wanted the blood to not be too prominent.
Also, I changed the title from what it was orginally to PENELOPE AND THE BIRTHDAY CURSE, adding the protagonists name to the title. A friend has suggested going back to just THE BIRTHDAY CURSE and add "A Penelope Amour Mystery," but I think that sounds too adult like and I wanted this to sound YA/Tween fiction.
Those are some of the considerations for my cover that may help someone else on the self-publishing path.