Customised Prefab Icons

Hi there chaps

I've just started using Unity to create a 3D game with a world that will be made out of 3D blocks. So far, so standard. When I drop my fbx models into Unity, and then make prefabs from them, in the [PROJECT] window Unity makes little renders of the models and uses those renders as the icons. It would be splendid if I could alter those icons, especially as they don't look very much like the map tile I want to see (single sided polys not showing up in the render etc).

Is this possible in Unity? As far as I can tell Unity is either rendering them to gpu ram when the prefab directory is first shown or rendering them to another location (the meta file?), but I've not tracked down where it is.

I've tried searching for an answer on this and come up short - apologies if I'm asking something that is common knowledge.

Cheers,

Robin.