Are either you or your grandparents behind cg-nat?

This probably doesn't answer your question but if you built them a Linux home router then it could be a bastion and storage server of sorts. It just needs DHCP, DNS, NTP, kernel configured to forward, basic firewall. As a bonus you could help them block some malicious sites, especially those that target elderly. If that router/firewall had enough storage you could even have batches of pictures to automatically upload to different picture frames to change it up. Downside is you might become their internet tech support which could be problematic if they live far away.

Another option may be to set up a service on one of their laptops that pulls down files from a VM you rented on the internet and then uploads to the picture frame(s). Or have it pull images from a forwarded port on your home machine to an obfuscated read-only rsyncd or chroot sftp using dynamic dns if you dont have a static IP. Both rsync and sftp using the lftp client can rsync data with rate limits to not bog down their connection.

If they are behind cg-nat then you could use tinc open source vpn to connect any two devices to a VM or to your home if it not cg-nat. Tinc is slower than WireGuard but more flexible for this scenario and slower is good to prevent congestion for the grandparents. With a network-to-network VPN you could communicate directly with all the picture frames.