CartoMobile supports a variety of mechanisms for using image data as a Base Map (see our article on Base Maps), and there are a variety of commercial and free sources for these various mechanisms.
Recently, the folks at MapQuest have also made available an OpenStreetMap-style tile server using imagery from a variety of sources. Thanks to them for providing this service. If you want to make use of their service, follow these steps:
When you re-enter the Map, you will now have access to this layer, courtesy of the folks at MapQuest.
They also have a differently-styled street map available using their data, which you can enable by following the same steps as above, and using:
http://otile1.mqcdn.com/tiles/1.0.0/osm
as the URL