Google Earth Introduction: Images

Importing an image overlay: You can download an image such as a scanned map and import it into Google Earth even if it isn't "registered" (doesn't have built-in locational information for the program to automatically overlay it in the correct position on Earth).

The image will drape over the landscape. There are tools for resizing, moving, and stretching it to closely overlay the location in Google Earth, and you can change the transparency to see the Earth layers through the imported image.

See the tab in this guide titled "Download Overlays" for a few sources of both registered image overlays as well as unregistered scanned maps that you can download.

Importing GIS (registered) imagery: You can import GIS raster imagery that is geo-referenced or registered, that is, includes information that Google Earth can use to overlay it in the correct position on Earth (Google Earth Pro and Enterprise only).

• GIS images must include projection information.  Sometimes the image is projected but lacks the associated projection file. You may be able to use GIS software to create the projection information.
• Google Earth then attempts to reproject the image to a Simple Cylindrical, WGS84 coordinate system (a.k.a., unprojected latitude/longitude coordinates).
• As of version 4, files using the NAD83 datum are not supported by Google Earth (GIS software could first be used to reproject an image).
• Google tutorial on importing geo-referenced raster data.

Example is from one of the historic topo maps from Google Earth Library.