Think about this; the cover of your book will not only be the main picture of your full-size book, but it will also be made into a tiny thumbnail size on Amazon (or whichever outlet you choose). Your book will be sitting next to hundreds of thousands of other titles, and you want to make sure that the possible buyer can clearly read the title of your book in the tiny form it will be displayed in.
This means keeping it simple. A large, simple font, possibly making the key word(s) bold, will make it easier for the reader to comprehend what your book says. The same goes for the design on the cover of your book. If there’s too much going on, the reader will get confused trying to figure out what they’re looking at and quickly resume their shopping on to the next item.