2483e263_TCL_TK_Command_Reference_Guide__Configure_Option
[Resource Name] tristateImage [Resource Class] TristateImage [Description] (Tk 8.5+) Specifies an image to display (in place of the image option) when the radiobutton is in tri-state mode. This option is ignored unless the image option has been specified. [Configure Option] 
-tristateimage image
TCL/TK Command Reference Guide