The possibilities are really endless when you have powerful software and someone who knows not only how to use it, but how to make ideas come to life visually.
I can appreciate the fact that you want to customize the covers to make them uniquely yours. Often times in my own pursuits I feel like I'm the only one who truly knows how to express my ideas from conception to completion so I am often hesitant to allow someone to help me. I know the feeling. With that being said though, a good graphic designer will be worth the cost every time.
If you allow me to I'd like to explain why.
If I had a baby and he had come down with a terrible illness I would without question take him to see a doctor and in the same situation I'm sure you would as well. The reason is that these are professionals and no matter how much I much I read on how to solve this myself I'd rather trust the health of my baby to someone who has dedicated their lives to their profession.
Graphic designers are no different. In this case, your baby is the book you've authored. Like any good parent you don't want anyone or anything to touch it. I don't blame you, you should have a significant hand in how your baby turns out. When it comes to exposing it to the world and allowing to impact people, you have to trust the professionals though. The survival of your baby might depend on it. Reading something like the Non-Designer's Design Book will allow you to have a small understanding of what it means to design, but will only scratch the surface.
We're in the fairly awkward position as graphic designers that every non-designer who looks at our work says to themselves, 'meh, I could probably do that' not knowing the decisions that went into it or the problems to wade through. The contemplation of how to connect to a specific target audience. The typographical choices that resonate with the content.
I'm not saying it's not possible to do it, I'm saying if you really want a professional result, you'll have to trust a professional. Great graphic designers know how to translate your vision and empower your idea.
You've expressed that cost is a big factor I would argue that the biggest cost is what it might cost you to have a cover that fades into obscurity and doesn't catch anyone's attention.
After all that, if you still aren't convinced, maybe this is something that you yourself want to pursue anyways. Designing is an awesome experience so I recommend that as well! In a roundabout way of answering your question what you want to do is possible through many photographic editing software programs. Chief among them is the powerhouse Adobe Photoshop. There are many great tutorials, books, and screencasts to learn the basics of Photoshop. There are also free alternatives to Photoshop such as Gimp. These software programs allow you to take photo stock and combine them in ways to make an incredible book cover-- this, among a vastly huge array of other graphical things.
I recommend looking into these programs and the large amount of content written about these programs if you choose to create your own book cover.