Well, there's not much to say.

About a decade ago I had a 24x7 linux based fileserver / LDAP / NFS / mp3 jukebox / misc box available at home which had nothing plugged into the VGA out... so I installed "zgv" (which is still available) because it is a console mode graphics viewer which can do slideshows. So I didn't have to bother with all of X on what was fundamentally a home fileserver. I had a very simple shell script to clean out a directory, wget pictures from all over the net (I had the local wx radar, and street scene webcams in Ireland, all kinds of stuff like that) and dump all the downloaded files (including 404 errors and the like) into the directory. Then I ran each graphic file thru a processor mostly so it would eat 404 errors and failed downloads and the like so they disappear rather than mess up the slideshow also to resize to the proper res. Then zgv in slideshow mode would display each pic for X seconds, and do it Y times, such that it took about 15 minutes to run, or maybe it was a half hour. Then rinse and repeat forever. Even with some abstraction and file renaming to force the order in the slideshow, we're talking about a "two screenful" bash script, it wasn't much.

The analog VGA output was fed into a gadget that converted certain VGA resolutions into composite video (This is why I was using imagemagick filters to resize the images, my converter didn't work well at certain SVGA compatible resolutions which "zgv" would use..) That composite signal via some modulators went all over the house. There's a lot more to that story. I basically had a crude cable TV plant in my house. A handful of highpass/lowpass filters and some cheap composite to NTSC modulators costs less than you'd think.

If I had to do it over again I'd probably steer toward X windows for the graphics, I'd bother to actually figure out how to auto-start the system rather than log in by hand at each (rare) reboot to run the script (probably outta inittab, errr.. systemd I guess). Given modern screensavers and the like it might amount to just boot up GDM/KDM/somethingDM and let a script update the screensaver directory of pictures... I have not kept up with modern FOSS developments WRT digital picture frames, the whole software setup might just be an "apt-get" away now. Or if not, it should be. Some double buffering so I could download and process the next set of "slides" such that the transition would be smooth and instantaneous when it updates, would be nice, and probably not too hard.

Color eink is not really available. I have a small BW eink shield and vaguely postage stamp sized display for an arduino. The price was unpleasant. I thought it humorous that to show the durability and no-power required of the e-ink they ship it displaying some Chinese characters rather than blank. At this time I think we're stuck with LCDs although Amazon, with its special history and relationship with e-ink could probably sell the worlds first actually shipping color e-ink digital picture frame. Which is my suspicion about the whole "amazon art" thing.

I want a "huge" picture frame. Not a little commercially available thing or even a hacked up laptop. So I'm probably stuck with TVs/Monitors (not much difference anymore) for now.