Terrain texture thumbnail is invisible

I picked up some nice new terrain textures to paint with in Unity. (I'd been using standard assets.) They work just fine, except for one thing: in the terrain gui, where it shows pictures of all the textures assigned and lets you pick one, my new textures are invisible. It's a little list of blank icons.

I can still select them and paint with them, but as you can guess, that makes it really hard to keep track of which is which! It also makes me suspect I'm missing something important. I tried making the texture settings exactly the same as the ones for the stock image (e.g. setting the Texture Type to Advanced and Mip Map Filtering to Box), but none of that seemed to do anything.

What am I missing?