A few things to try, none of them require knowing anything technical. But some research indicates that Safari handles and loads images differently than Chromium-based browsers and Firefox. Found some info online I'm copying here that could potentially help.
1: clear Safari's history & cache. Safari menu → Clear History → choose "all history." This logs you out of stuff and forces Safari to refresh everything. If the images come back quick after that, your cache had gotten gummed up. (You'll need to log back in here.)
2 (content blockers): Safari → Settings → Extensions. If there's anything listed, try turning them off and reloading the forum. Some ad/tracker blockers get aggressive about image-CDN domains.
3 (icloud):Safari → Settings → Privacy → uncheck "iCloud Private Relay" temporarily. That feature routes traffic through Apple's servers and can slow image-heavy pages.
Fourth, make sure your Mac/iPad/iPhone is on the latest OS update. Safari ships with the OS, and older Safari versions handle image loading worse.