Can anybody help?

Fixed, thank you!

Okay, so I'm posting this here in the hope that someone can tell me what I've done. I was making icons yesterday and I have no idea what I did, but I somehow seem to have managed to replace the layer thumbnail in the layers palette with a black and white picture of a paintbrush rather than with a picture of whatever's in the layer. Have to admit, it's the single most annoying thing and I want to undo it, but I can't figure out what I did in the first place.

Anybody know how to fix this?

The layers palette looks like this:-

Photo Sharing and Video Hosting at Photobucket

I realise this isn't strictly icon related, but I have no idea where else to ask this question. If I can't ask this here, feel free to remove this post or ask me and I'll take it down.