Bump.

In the Editor, click Settings, then click the button that says Refresh Thumbnail. I recommend waiting until you've got most of the walls, if not all of them, already done; creating the thumbnail at the end.

Also, it seems to only work once per map, per game session. So if you make changes and want to refresh the thumbnail after having already done so, save the map and completely exit the game. Afterwards, launch the game, go back into the Editor, load that map, then click Refresh Thumbnail for an updated version of the wall preview.

I think enabling Dev mode in the options.xml file will allow you to update it on the fly but I haven't tried that yet, but I should.

But for best results, I suggest using the generated thumbnail as a starting point and cleaning it up in a program like Photoshop for quality purposes. Sometimes walls get thinned in certain areas. Maintaining a three pixel width for walls usually looks best.

The filename will look something like mapname_thumb.tga