I often find embedded images from other instances straight up don't appear, while embedded images from lemm.ee take a really long time to show up (and sometimes don't at all)
For example, the top comment here has an embedded image but it's not showing on lemm.ee: https://lemm.ee/post/38434336
An even weirder thing is that it thinks youtube links are also images, and tries caching them.
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If that's the case why not add some logic to fallback to not proxying the image when it fails to download or some other issue occurs preventing the image from working correctly on the image proxy