code: public static String getProcessDiagramResourceNameFromDeployment(
        ProcessDefinitionEntity processDefinition, Map<String, ResourceEntity> resources) {
      
      if (StringUtils.isEmpty(processDefinition.getResourceName())) {
        throw new IllegalStateException("Provided process definition must have its resource name set.");
      }
      
      String bpmnResourceBase = stripBpmnFileSuffix(processDefinition.getResourceName());
      String key = processDefinition.getKey();
      
      for (String diagramSuffix : DIAGRAM_SUFFIXES) {
        String possibleName = bpmnResourceBase + key + "." + diagramSuffix;
        if (resources.containsKey(possibleName)) {
          return possibleName;
        }
        
        possibleName = bpmnResourceBase + diagramSuffix;
        if (resources.containsKey(possibleName)) {
          return possibleName;
        }
      }
      
      return null;
    }
docs: Finds the name of a resource for the diagram for a process definition.  Assumes that the
  process definition's key and (BPMN) resource name are already set.
  
  <p>It will first look for an image resource which matches the process specifically, before
  resorting to an image resource which matches the BPMN 2.0 xml file resource.
  
  <p>Example: if the deployment contains a BPMN 2.0 xml resource called 'abc.bpmn20.xml'
  containing only one process with key 'myProcess', then this method will look for an image resources
  called'abc.myProcess.png' (or .jpg, or .gif, etc.) or 'abc.png' if the previous one wasn't
  found.
  
  <p>Example 2: if the deployment contains a BPMN 2.0 xml resource called 'abc.bpmn20.xml'
  containing three processes (with keys a, b and c), then this method will first look for an image resource
  called 'abc.a.png' before looking for 'abc.png' (likewise for b and c). Note that if abc.a.png,
  abc.b.png and abc.c.png don't exist, all processes will have the same image: abc.png.
  
  @return name of an existing resource, or null if no matching image resource is found in the resources.