1. Create an Image Overlay: Right-click "My Places" and select "Add > Image Overlay". Give the overlay a name, and then either put the URL of the image in the Link box or click the Browse... button to use a file from your computer.
2. Position the Image: On the Location tab change the borders of the image so it covers the whole globe, i.e. North: 90N, South: 90S, East: 180E, West: 180W.
Turn off any layers that will interfere with your map, such as roads, borders, labels, terrain, etc. If the borders don't line up with yours, you probably have a map projection other than equirectangular.