The new value may be null, to simply remove the icon. If non-null, the icon object should have the GoObject.Selectable property set to false.
Remarks
Instead of setting the icon after creating a node, you may find it easier to override the CreateIcon(ResourceManager,String) method(s). The new shape will have its Center location, Selectable, Resizable, Reshapable, ResizesRealtime, and Shadowed properties copied from the old shape.