Domain Specific Language tools

I've been learning how to use the Visual Studio DSL tools to model an architecture language called 'ArchiMate'.  The DSL tools allow you to assign custom shapes using the DSLDefinition.ImageShape class, which is described in the documentation as supporting several image types, including WMF.  However, once I assign a WMF resource to the ImageShape.Image property the DSL fails validation and won't compile.  I found that I could change the resource manually after running the TextTemplate generation by direction modifying the 'DomainModelResx.resx file. 

 Now my problem is I want the image to be resizable like the stock GeometryShape class.  There doesn't seem to be documentation on the methods to override, however.