For a Photo Library, I want all users to be able to download photos but not add/delete photos. A few users will have full control, but everyone else should only be able to use the photos without changing anything else. I keep trying different permission levels, but I can't get it right. Any ideas on how to basically make it so visitors can download photos (not just right click and save) but not do anything else?
By giving all users read access, they will only be able to download/view photos in the library. You can do this by clicking the cog > Library settings > Permissions for this library, then adding users to either a group with read access or configuring unique permissions if this library needs different permissions to the rest of the site. Here is more information on configuring permissions for a library:
You can refer this article for more information about User permission: