I don't have expertise in this area but I have been researching something similar with music. There are companies that have large collections (millions) of copy written material and you pay a small yearly fee to get a license to use it. This is done a lot with music and karaoke. You need to buy a license to cover the royalties.
If the above is a no-go, you can always use Getty images, or search for images that are party of "public domain". That may actually be easier from the get go.
