On some communities the header image and community logo is broken, e.g. here: https://lemm.ee/c/idm
I’m the mod there, I haven’t changed images since I started the community, a year ago. If I click on the Edit community button, I can see both images on the sidebar.
If I comment somewhere and link to external image with the markdown syntax  images not showing up. This works in post bodies, it only affects comments.
Created a test post here, you can see the image in the first comment is not visible: https://lemm.ee/post/38021843
Edit: This may be related: https://lemm.ee/post/37717282
We had a temporary issue with generating different formats/sizes for existing images. I believe the issue is now fixed, our server is going through a backlog of missing files currently and regenerating them. I believe all broken images will be fixed within the next few hours.
Sorry for the inconvenience!