It's magic! No, really . . .
To be honest, I cheated a bit. The background file is specified by a path. Normally logon come with their images, so the path is just "X.jpg". However, LogonStudio has the ability to load files from any path, including paths relative to environment variables and executable resources. As all of the Windows wallpapers are in predictable locations, I just have to point to them.
The skin file itself is tiny, as the 10kb autogenerated thumbnail is not included in the exported skin.
It will just skip the logon if the file isn't there (for example, the Media Center logon is only available if you actually have this Premium feature installed).