[JustinThomason]

My thought on how overlays might work would be to have the ability to add additional image files with a transparency layer (e.g., PNG). As long as the new image is the same resolution as the base image, it should work to turn that secondary image on or off - at least in theory.

My thought on application would be to use a program like GIMP or Photoshop which allows the creation of layers to make the individual overlays as separate layers, then export each layer as it's own PNG with transparency.

For instance, you could start with the base map with all physical features, then add things like political boarders in a layer. If you export the "Political" layer as a discrete file with the transparency intact, you could add that to the smart image as an overlay, and you'd be able to reveal or hide political borders by turning the overlay on or off.

Obvious caveat - I am not a programmer, so my "simple and straight forward" solution is certainly anything but. However, from a practical application/usability standpoint, I don't think it would be all that convoluted a function to add.