I opened WorldPainter today and it said there was a new version, and would I like to update? Sure, why not. It will over-write your files, it said. Fine, that sounds nice and tidy to me! Then I opened the program again, and all my “Custom Brushes” were gone.
OK, no biggie. I’ll download some more. Next time I’ll save back-ups, too!
Here’s a nice post at the Minecraft forums, explaining “Custom Brushes” and giving some resources. There’s links to a tutorial not to mention a couple of brushes that you can download from that page.
I also searched Google Images for “geometric fabric patterns” and will try making Custom Brushes from some of those.