They can be saved on a thumb drive

Apart from the 15,000 items on the primary purpose with multiple images on many. Then the ancillary historical items of interest that more than double that. Curating is a process, and surprisingly well run given we are all volunteers, it has to be with Charities.

No, thumb drives are silicon NAND which has a nebulous lifetime, SSD is still NAND with a processor monitoring and correcting lifetime reliability (only when powered) but there are limits, and rotating disks are mechanical.
As delineated in this parish many years ago, a member quoted his 10 identical "disks" he stored duplicated data on. He found 5 had errors after 5 years. Silicon is no better. After 5 years, with no errors, you are on borrowed time. CD/DVD? Forget it!
The real problem is telling this to the proverbial "cats you are herding".
Knowing where those errors and the mechanism to correct, is a whole other. Best solution is to pay for guaranteed storage on-line. Money being the "only" problem, apart from the herding issue.

Fakebook probably have all the images offered to them (E&OE). But finding, let alone curating, is ............... just not. I have seen glitches where posts/images have been perfect for a few hours, then it goes slow then the posts vanish (think hacking). Hardly a secure repository.