Same Pb here, like @OP and everybody in the Thread, on 'CS' + some "other" Site (that I don't want to name), and both Sites are based on phpBB v3.x.
And I can also reproduce on 'Dragon Cave' indeed, but 'dragcave' doesn't seem to be based on phpBB...
'CS' allowed me to "Add a Certificate Exception" as the only Site, the 2 other ones not, and after adding an Exception for 'CS', I can use the Site a bit (and post here), but like @OP, Images are not working "very" well, for the 'Pound'/'Lost and Found' and for the Archive(s), both for Pets and Items.
=> The first time I load such a Page, "some" Images are still loaded/visible, but if I reload/refresh the same Pages, all Images disappear one by one and get replaced by "Pet" or "Item" in Text.
EDIT_1: Also same Pb with 'ELD' ('Eldemore'), but 'ELD' like 'CS' allowed me to add an Exception...
EDIT_2: Same Pb also in Trades, about Images, I only see "Pet" (in Text) for all Pets, ah-ah...! Not very handy, ah-ah...!
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Browser I'm using is 'Pale Moon' (v26.3.3), on Win10_Pro_x64 (v21H1).
Everything (all 3 Sites) works fine on the other hand in Firefox (v55.0.3), same OS.
(I also have 'Chrome' (v76), 'Edge' (v44, I think, not 'ChEdge'), and FF (v92, I think?, Latest), but I didn't check in those...)