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I was able to reproduce the issue on old.lemmy.world, except that I don't get "Save Image As..." in either Chromium (I don't have Chrome) or Firefox. Poking around in the Developer Tools, it looks like old.lemmy.world displays images by setting them as background images for some reason:
I would consider that a bug, so you might consider submitting an issue to whoever's responsible for old.lemmy.world (maybe start by posting in https://old.lemmy.world/c/support@lemmy.world).