Had a look. Seems that the thumbnailer got taken out mid-job by a freak accident that killed the worker process that had responsibility for it. When the thumbnailer fails, you'd expect LynxChan to abandon the thumbnailing job and use a generic thumbnail, but instead it apparently refuses to move the main file concerned to the database... yet still creates an empty entry for the media, meaning any future posts of the same file reference the empty media entry and there's no way to fix it except by deleting the empty entry on the back-end and THEN re-uploading the file. That seems to work.
The good news is that it was a freak accident and shouldn't recur. Bad news is that StephenLynx is... well, you know the rest.
Sorry for the trouble, folks.