App to use iMac as digital photo frame for photos on a network drive?

Solution 1:

Nobody volunteered a solution, so here's what I did...

• Screensaver: I used the in-built screensaver, with a few changes (see below)
• The Default duration of the slide show was too quick (3 seconds), so I used OS X Mavericks screensaver changes pictures too quickly: How do I slow it down? to fix it. Note the special csrutil instructions since Yosemite.
• Network drive: I mounted a network drive, then changed the Login Options to permanently mount it. See: How to Automatically Connect to a Network Drive at Login in OS X
• On/Off control: I used the Energy Saver control panel to Wake & Sleep the machine at set times. Not as good as sensing light/motion, but it will do for now.