I am running a scenario similar to that described in your Developer's Guide (using an Image control in a DataTemplate to have an image drawn for each MapElement).
It works like a charm as long as the Image's Source refers to a local image ... however, when I change the Uri in the image to that of something globally available ( http://g0.gstatic.com/ig/images/weather/mostly_sunny.png for example)
I get the dreaded "element is already the child of another element" error.
Never mind - used a smaller image and it worked.
Actual problem: bad URI supplied at first, really just a very bad error message which put me on the wrong track.