old thumbnails can't be removed

In "~/.cache/thumbnails/" there are two directories: "fail/mate-thumbnail-factory" and "normal" that hold thumbnails. I've got a handful of old thumbnails that are somehow "stuck" in these directories from at least as far back as 2013. I've deleted them numerous times, but they reappear after each boot. Where are they even coming from? They're like ghosts. How do I remove them permanently?

I'm using Debian Buster at the moment, but they have been there since perhaps Sarge.

Bob