A snapshot of the surface module window can be taken via the command
docVMR.SaveSnapshotOfSurfaceWindow("C:/MyData/myMesh");
This saves a snapshot to disk of only the surface window itself, not of the BrainVoyager QX Global Tools window and the menu.
It is not necessary to add an extension to the specified image name. The program will take the default image type that is specified in the global preferences dialog of BrainVoyager QX (accessible via File > Preferences):