I've got an example on my Sample Application, showing the use of custom icons on a tree, where the image name should be prefixed according to it's location. For example, a standard image would be referenced like '#IMAGE_PREFIX#name.gif', a workspace image like '#WORKSPACE_IMAGES#name.gif', and an application image like #APP_IMAGES#name.gif. Our documentation is lacking on this front, so we'll ensure it gets updated for a future release. I hope this helps.
Thanks a million, Hilary. It works like a charm :-)