A class has been created in the images folder to make accessing images easier. Open the ImageUtil.java file in org.netbeans.modules.uml.resources.images package. You will quickly get a feel for the methods that can make image access simpler. If you have a need for a different method to access the images, please add to this class.
For element type to image file mapping, the technique is still to use the CommonResourceManager which has a large map of element type names to bundle keys which then map to the image filename. This should be reimplemented to remove the need for the large in memory map.
For diagram images, there is a method to retrieve the diagram's image file name and another method to retrieve an instance of the diagrams Icon. All that is need is the diagram type (int constant from IDiagramKind).