When you post an image, if it's wider than 1520px, the forum automatically generates a preview image for the post that's smaller, providing a link to the original image.

What I think happened is a bug (?) in the forum job that looks for 'orphaned' uploads (uploaded files from removed or cancelled posts) and deletes them.The job erroneously removed the preview images and thus the missing image in the posts (clicking on the link would still show you the original image).

@BuZz restored the preview images, for now I've disabled the 'cleanup' job until the issue is solved.