Preview Pictures on facebook when linking to permies threads

I have no idea if there's a way to fix this, but I've noticed that when I post a link to a thread on permies, the little preview picture has nothing to do with what's going on with the thread. It's usually an ad for Rocket Mass Heaters or something. Which is nice and all, but probably doesn't entice people to click on the link I posted. It'd be great if it took a picture from the thread, like the programming does for the preview pictures here on permies.

If you link to the first post in a thread rather than the thread itself, it will usually pick an image from that post rather than from the ads above.  I've not completely cracked it though.

I actually linked to a specific post in that thread, and the post had pictures in it .