You may be familiar with Google Earth image overlays. Somewhat less popular but no less cool are screen overlays. These are images that appear in a fixed location in the 3D viewer above imagery of the Earth. In other words, as you navigate or fly across terrain, the screen overlay graphic remains visible in the same location of your screen. Screen overlays also work in KML files viewed in Google Maps. Some users have used screen overlays to brand KML with logos or to provide a map legend to their KML content.
Note that you cannot add a screen overlay directly in Google Earth; you must edit the KML directly. However, this is a simple process as described in the KML Sample document and reference guide.
Google Earth Lessons has a few interesting examples of this.