For jBPM 4.4, you need to deploy the image together with the process definition (as far as I know with the same filename, but with the png extension). When executing, you can retrieve the currently executed task. You can then retrieve the coordinates of the activity using the
getActivityCoordinates(java.lang.String processDefinitionId, java.lang.String activityName) from the ProcessService class.
Note that you must have created the diagram with one of the editors used for jBPM (either web or Eclipse), because the editor stores the coordinates of the nodes in the xml.