If you want to see the diagram, you currently need to add the png to the application classpath (with the name {processId}.png.
For example, you can find the com.sample.evaluation.png in jbpm-console-server.war/WEB-INF/lib/jbm-gwt-graph-*.jar
So you could add it there or any other location that is part of the application classpath.
I'm finishing guvnor integration, that will allow you to put all your files (process definitions, images, task forms etc.) in a guvnor package and simply let the jbpm-console get everything from there.
Kris