It occurred to me that yesterday there were probably some who said "Yeah, easy for you to say, you have Photoshop!"
There's nothing easy about Photoshop as everyone well knows who's been here since the blog's inception. But yes, if you don't have a copy and cannot for the life of you figure out how to get a copy, here are a couple suggestions. GIMP is a freeware graphics program that will make it possible for you to build a cover. For a smallish amount of money, compared to Photoshop, Corel Draw is excellent and is quite sympathetic to newbie users. Shop around for a legacy version and you'll be very pleased.
Once you know a couple common techniques, using these programs will enable you to create the same kind of covers you see on traditionally published books. I do advise you to learn the whole layer thing. You might not know what that is now, but once you start learning the program, you will be confronted with it Layers are your friends. They will isolate potential mistakes and make changes simple.
Cover images for ebooks are still in their infancy. I expect that over the next year they will become far more sophisticated and designed more for the thumbnail view. Tradpub books are designed for bookstores. That they are displayed at BN or Amazon, is an afterthought. This will change. So you do want to be aware that anything you design must be understood in a quite small size.
I think I probably failed that test with Virgin Warrior. There's an awful lot of detail in the image and the text is fairly small in thumbnail size. But it was a trade off because in the large size, it all looks swell. Like anything, it was an artistic choice as well as time and energy investment. If it was a book, I probably would have done it differently but since it's one story, I was willing to do halfway there.