I have a Mac Air running ios 10.11.6 & Photos 1.5. After much experimentation, I think I have discovered something of which no one seemed to be aware when I originally asked the question. In case someone might benefit from my effort, here are the details of how to set up a wallpaper slide show of all or a large subset of your photos:
- In Finder, Pictures, create a new folder such as “For Wallpaper”.
- Open a new finder window
- In the new window, select Pictures, then Photos Library.
- 2 finger click on the large Photo Library icon
- Select Show Package Contents
- Click on Masters
- Keep opening folders until you find the desired photos
- Drag & drop the desired photos into the “For Wallpaper” folder
(if Photos app is open, close the app to complete transfer)
9. The photo files now reside in “For Wallpaper” folder but no longer in the Masters folder but the photos still appear in Photos app & in their original Albums.
10. Go to “Desktop & Screen Saver” in System Preferences
11. Select Desktop at top of window
12. Click the + close to the bottom left corner of the window
13. Select the “For Wallpaper” folder
Your wallpaper should now be a slide show of your desired photos