How do I move a picture on my Mac to become part of Screensaver

I have my screensaver (System Preferences>Desktop & Screensaver) pointed to a folder on my mac, so any photo in that folder can show up in the screensaver. Are you using a specific folder for your screensaver pictures? If so, you can get pictures from your Photos library into that folder by selecting the pictures and then choosing "File>Export>Export 1 Photo" (or more photos if you choose.) When you then hit Export, you'll be asked where to, and you can choose the folder you're using for your screensaver pictures.

Alternatively, if your pictures are in Photos, in your System Library (identified in preferences), then you can go to System Preferences>Desktop & Screensaver and set the Source to an album in Photos. You can make a special screensaver album in Photos that holds the pictures you want. In Photos, pictures can be in more than one album.