It was very simple. Someone embedded an 100 k image from on old game on a forum post.
Because it was embedded, the image was being served from my site. In two days, it had about 4,000 to 5,000 downloads. While my site does have the bandwidth to serve it for a while, I'm not inclined to be nice when there are no visitors from the forum to my site. I can see that all the hits were just the image - absolutely no one visited my site from that. Not even the review the image is linked to.
So the image has been renamed. When things settle back down, I'll change the name back.
The trick is definitely that I just don't want to find my site runs out of bandwidth for a month because someone embeds a game image. I would not care if they made a copy of it. It's an old game, it's an old review and there's nothing particularly tricky about the screenshot.