I hope I don't clog up the thread with a bunch of images, but I tried what you suggested. The issue seems to be with Brave browser.
A lot of images
First I tried incognito mode on Brave with shields down. This is what Risa looks like:
I try to expand the thumbnail of post nr 2 by clicking on the thumbnail and nothing happens:
I click on the title to open the post, and click again on the thumbnail to expand it:
I click on the post title again so it opens the link to the image in a new tab, and it works!
Now I try the same in normal mode, so not in incognito. Again, Brave shields are down. First I clear all browsing history like you suggested. Risa:
Opening the same post, click on thumbnail to expand it:
Click on post title to open it in new tab, no image but the same url as in incognito mode:
I tried some different things, and it seems the issue lies with Brave. On Firefox it works, as long as you disable uBlock origin. So I could use a different browser and solve the issue, but I assume there's other people using Brave that want to view the images so I hope this gets solved eventually! Edit to add: I only have this issue with communities from startrek.website, the little icons next to community names or usernames are broken as well. I can easily find Risa in my list of subbed communities because the little icon is broken.
If you expand the images in the spoiler you can see screenshots of me trying your suggesting about the cache. I cleared the browsing history (all of it, not just cached images and files) and it still doesn't work, and tried incognito mode like you did, and it worked a bit because I could at least see the image if I opened it in a new tab. I'm adding the info about adblocker on/off because that's the only 'fix' provided by the admins on Mastodon and in the FAQ. Since that's the first thing I tried to disable, it's obviously not that hence me coming here to ask for help. I'm trying to be as complete in my info as possible so the person who has to solve it hopefully has the information they need :) and as I'm not that much of a computer person, I don't know what information is most important to share.