I have decided to self-publish some short stories I wrote years ago.
I understand the importance of a good cover. I have known for years that “Don’t judge a book by its cover” does not work in the real world.
I don’t have a big budget but I have access to stock photos. I have some skills in manipulating them to look different from the original (including using layers).
I am wondering how good are stock photos as book covers?
That depends on the license of these photos. If you merely “have access” and no publishing rights, expect a call from lawyers and shelling out a rather hefty sum to settle it out of court… If you can purchase right to use given images, check the license you’re purchasing. If it lets you modify the image, okay. Often the author doesn’t agree to that – you’re allowed to change format or color balance but not outright edits like combining images through layers.
OTOH, many people definitely overestimate their own skill at making graphics. It’s easy to make a book cover. It’s very difficult to make a decent book cover. And it seems you’re a writer, not a graphics designer. So, whatever you do, once you have the project ready, go ask some professional designer for opinion, and accept any criticism – especially negative.