Issue
There’s a significant problem at the moment: anyone can upload any content. While this might not seem problematic initially, it opens the door for someone to upload illegal or harmful material, like "cheese pizza." If others unknowingly come into proximity with such content, it would be automatically downloaded to their devices, potentially putting them at risk of being flagged by authorities or worse. Although it's the responsibility of the server administrator to prevent such content from being uploaded, the way the mod currently operates makes it nearly impossible to enforce this.

Solution
The local caching system is useful, but it should be designed to automatically clear itself when the player leaves a server (or world). Only the files that the user has personally uploaded should remain. There could be a configuration option to disable this behavior, but by default, the cache should be cleared upon disconnection.

I run a pretty large server and this is becoming a big problem.

I will check regarding a flag, though it won't be enabled by default. The cache is here for a reason. You will also encounter the same problem with logs, cached chunks, cached player skins, etc.
If you want to reduce the risk as a server admin, reduce the valid resolution to pixel art and disable showOtherPlayersPaintings.