There are some issues writer’s need to understand about artwork they use to make their book covers. Just because a site says the artwork is royalty free doesn’t mean it is. People put images they find onto sites and say it is free to use but they don’t own the image so, while you might think you have something free it also might come back to bite you in the butt if you don’t have permissions from the creator or company that owns the copyright. Even if you think you are free and clear it can still result with you owing money to people you never knew existed.
A lot of sites offer images for purchase and you might think you are free and clear with those as well. Fact is you have unlimited digital distribution with most of those purchases but limited physical printed versions for sale. Many have a limit of 1500 or 15000 physical printed copies before you have to pay for an expanded license from the owner. This can end up being problematic if you don’t know about that little clause and the owner comes back and sues you for copyright violation after you surpass the limited use mark.
Most limited use contracts have a clause that allows you to use the image(s) purchased in composition of digital artwork. This is the sweet spot for writers. If you get an image, clip out the subject, add it to a digital background and modify it to be part of a larger piece (i.e. a cover design) so that the original piece comprises no more than 20% or so of the whole or that you have modified by 5 steps of digital manipulation, you have an original piece of artwork that you own the rights too.
Look into the copyright license of any image you use. Read the fine print and make sure you know what you can and can’t do with it. If you just work off faith that someone on some website knows if the image is free, you may end up paying much, much more than you would have getting a custom design done to start off with. Even some of the book cover service sites just use bought limited use images. Make sure to ask for the full copyright use agreement. You want to sell a gazillion books? Make sure something small like cover art is not going to cost you big time down the road.