It would be one of my big wishes at Graphisoft to build a simple possible way to add pictures to existing elements - because if you are working deep in the developing process - it is simply not possible to know all pictures you will need later in Your new gdl - UI.
The ideas for the IU are coming later in this programming-process.
There is a possibility to add pictures later over xml-converter - but there is not much "support" ore "documentation" from Graphisoft at this option - for simle minds like mine.
There should be some sort of button in the GDL-programming enviroment to add pictures directly!
Also it should be possible to delete inserted pictures - when not needed any more (to save ram) - some sort of picture-library editing..