About randompixbot, I can certainly get something up & running quickly, but. .
. this is all getting a bit complicated. Not impossible, but not sure it's worth the effort.
How about something simpler?.
Proposal 1: randompixbot will post, at certain intervals (hourly? daily? something in between?) a random oldpix, unconditionally.
Beside this, people can post (at a certain schedule, with the usual first to rONG wins; or just whenever they feel like it) a favourite frame. (People aren't good at random .).
Proposal 2: we decide on a schedule for posting rONGs. For instance, hourly, or daily, or something in between. Randompixbot will run N minutes later (could be 10, as it is for newpixbot, or 25, 30, whatever we decide), check if any.
oldpix was posted since rONG time, and if not, rONG a random oldpix.