App to use iMac as digital photo frame for photos on a network drive?
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 &amp; Sleep the machine at set times. Not as good as sensing light/motion, but it will do for now.