I charge the listed, one time fee, for each cover. How I obtain the images I use for covers is detailed below. Basically, unless you think your book will sell, or will be downloaded more than 250,000 times, you have no concerns.
I get images from a few places. And every image I use is either covered by a Standard License or a Creative Commons Zero License.
I have subscriptions with 2 major stock image sites. Any image that has a recognizable face will come from either of those, as they keep model releases on file. Standard licenses apply to those images. I also get images from free stock sites, but not any that have recognizable faces. These are free for commercial use. And FYI, since images that shouldn’t be on these free sites somehow end up there, I am cautious of any image I select from them.
Any cover with a SKU starting with SL is covered under a Standard License. This license covers a maximum of 250,000 sales or downloads. Any cover with a SKU starting in CC is covered under the CCO (Creative Commons Zero) license, meaning free for commercial use, and are not limited to the 250,000 maximum mentioned above.
If this doesn’t answer any questions you may have about licensing, please feel free to contact me.