Some bots posted lots of illegal pictures in the https://lemmy.world/c/lemmyshitpost community, and because of federation, those pictures have spread to all instances, including this one.

The lemmy software doesnt have good moderation tools for abuse like this, and the quickest way to get rid of them was to delete all cached images for the last couple of days.

You may see some thumbnail images missing in the web interface, but I personally dont see any missing images in my mobile app. I guess it has its own thumbnail cache.