10.12- Screensaver question- Photos not a choice
I have just spotted an older (2012) answer at this link:
http://hints.macworld.com/article.php?story=20120112094116261
However it suggests clicking and holding the "Screensaver" tab, which didn't work for me.  But clicking and holding the "Source" drop-down menu button did work for me - up comes an option to use "Photos" but if you choose that, you can then choose between the "Photos" app and "iPhoto" app. This is a better solution than the one I just posted, I think.