You can buy the rights to stock images fairly easily, but be sure you are buying the level of usage rights you really need. Don’t buy the rights for a web post and then use the same stock image for a book cover without going back and increasing what you paid for, as one example.
Also be aware that unless an image is new and you purchase exclusive rights to it, there is no way to know how many other people may already used it. (Purchasing exclusive rights is a good idea to reduce potential future usage by others.) It might be one no one else grabs, or yours could be one of eight new releases that month with the same image. This is true for any image that you did not personally take, but it is particularly common with stock images. (Hint: It it’s featured in “most popular”, a lot of people have already downloaded it.)
Very few people dig through the government archives to find images. This is probably in part because so few people realize they are there and in part because there is a lot of dull stuff to sort through to find the good stuff.
Canva (tecnically, more for creating graphics than a straight-up image source)
Creative Commons (use with caution)
Creative Commons is one site that does offer images that are free and copyright free. HOWEVER. There have been a few instances where an unscrupulous person puts images up and marks them copyright free but later removes that and sues people who downloaded them as “copyright free” and used them.
This is why it is extremely important to be very cautious when using “copyright free” images that are not marked that way by a reputable site, like Getty Photos or the Library of Congress. It is also the reason POV Press requires all authors to document where their images from and include proof that they can legally use them for the intended purpose.
If you choose to use one of these images, do everyone a favor and take a screen grab that shows it to be copyright free, preferably with the date and time also visible. If possible, you may even want to email the contributor to confirm usage rights. Keep that email chain safe as well. Keep that file in a safe place or two, and include a copy with your cover form for your final submission.
The Creative Commons terms are also used by other sites, such as The British Museum, below. The difference is that the main Creative Commons site is filled with images uploaded by people who then dictate what usage others may make of their images. They can change this at will. The British Museum is an institution that isn’t prone to playing games like that and is highly unlikely to change their terms to increase the restrictions on use.
These sites have “premium” paid options to receive a larger selection and a large number of free images that users have donated.
These are museums and other sites that have images that can be used for non-commercial and educational purposes such as non-sponsored blog posts, but they should NOT be used for cover art or other commercial purposes without receiving specific, written permission, unless an image is very clearly marked as having no restrictions on use.
The Photography Museum has a set of curated links to other sites. No one at POV Press has reviewed them for copyright status.