I found a blog related to this. Basically, I need to get (or create) a png (image file) and put it into the Guvnor package asset, or put it into the jbpm-gwt-graph-5.0.0.jar. I am going to test it using the Guvnor because the second option (putting into the jbpm-gwt-graph-5.0.0.jar) is not practical.
bwj