Re: changing icon shadow size when zooming

All of the information from the GIcon instance you pass into the
GMarker constructor is copied into the GMarker object. No reference to
the original GIcon object is retained by the marker.

I would suggest that you first try creating several GIcon instances
with different sized shadows, and create markers from them to see how
these markers look and feel on the map. Perhaps test the behavior when
you swap markers as the zoom level changes, rather than trying to
modifiy the marker images. You can do all this with the documented API.

Beware that changing the marker image after it has been created
requires that you descend into the undocumented depths of the marker
implementation details. This is unsupported and subject to change
without notice.