Hey, I found your post because I want to do the exact same thing. I post my photos to a web server I control, generating thumbnails and smaller versions, then create an XML file for use by XScreenSaver's Carousel module (here's a video of it (not mine): https://youtu.be/IyqWkGVrFIY
). XScreenSaver is open source, you might want to check that out: https://www.jwz.org/xscreensaver/download.html
or direct to the getimage script (not the official repo, just something I found): https://github.com/jay-snyder/xscreensaver-5.36/blob/master/driver/xscreensaver-getimage-file
Carousel is what I prefer for computers, but for a bigger screen attached to a Roku, I would want something like what the Roku and Apple TV already does, and XScreenSaver has those modules too, but I'm sure the downloading and processing of the images is all the same code in the background for all the modules. I am a developer as well, but my time is very limited. I will try and get back to you if I figure anything more out, but I figured I'd throw in my two bits and give you some encouragement.