More things I've learned today:

The SKB file is a backup, which is created every five minutes with autosave on. You can turn both the autosave and backup copy generation off (even independently of each other) if you want from Windows menu -> Preferences, and then choose General on the System Preferences dialog box. They can be "safely" deleted, if you don't want to go back to them.

You can create your own Materials from image files and save them so you can use in other models or share them just like you would SketchUp models (SKP files). This creates a SKM file, and the image you use to create the material is embedded within it, so you don't need to share the original image file as well or keep the original image in a fixed location in your directory. You can stick them in the appropriate subdirectory in your Google SketchUp Materials folder and they will show up as part of an existing materials collection (or a new materials collection if you want to create your own).

This opens up a world of possibilities...