The "Online Maps and Images" window shows images of the area around aircraft's current position. Unlike the "Map" window, these images are downloaded from various servers, so you need a running internet connection to use it.
More examples ...
In the upper left part of the map are some yellow areas that can be used to modify the map (just click on it with the left mouse button).
The [+] and [-] areas are used to zoom in and out.
[Map] shows a map of the area around aircraft's current position.
[Sat] shows a satellite image of that area.
[M+S] shows an overlay of a map and a satellite image of the area.
[Ele] shows elevation as colors.
Map images and overlays
Satellite and elevation images
Jet Propulsion Laboratory (http://www.jpl.nasa.gov/).