Thanks to a tip from reader @prcrouch at the Zengobi forum. This site has a very large collection of background patterns available for free download.
A couple of ways to use pattern files:
- Drag one of the .jpgs for a pattern into a map, and create a Picture Adornment
- Drag a .jpg into ~/Library/Application Support/Tinderbox/fill and it will then be available in the Edit Background dialog, accessed from the Map's contextual menu, as an option in the Fill dropdown list. "Fills" are tiled in the background of your map.
For advanced users, the downloads from WebTreats include a Photoshop .pat file for your own modification -- though this is not usable in modern versions of Photoshop.