This may not be an ideal solution but I'm dropping in to share this with everyone who's complained of the missing screensaver function (as I have). I had decided to just give up on Roku fixing this and go create my own (the myriad of available screensavers are developed by 3rd party folks using Roku development platform, which it turns out is fairly robust). I mean, I'm a software engineer by trade and a photographer in my spare time, so I got started and have been working on my own for about two weeks now.
ANYWAY... while researching how to make part of this code work, I found a user who has made and published a brilliant screensaver. It allows you to link the screensaver to your Google Photos account (or to many of them if you wish), and it will display your photos as your screensaver. He has put plenty of user-friendly options in it, too!
Go here to find it and add it to your Roku account:
Like I said - it doesn't fix the OP's problem, but it's a pretty good alternative!