The bottomline is don't post the photo(s) to your private tree to which you've invited people if you don't want them to see and/or copy your photos. If the tree is a "research" tree, I wouldn't put photos on it but keep them on my own computer genealogy program.
I don't know what you're wanting to do so can't advise you without more info. If you'd like to share photos with some, but not all, your invitees, the best way would be by email rather than posting photos to a shared tree. Even then, the person you share your cherished photos with could easily upload the photos to their own tree after getting them from you.
Hope this helps.