Hi Erich,
Unfortunately, there is no way to do that in GDL now. Not even in v15. It's a good idea though - it's worth considering for a later version of ArchiCAD.
Right now, you should slice up your UI pictures wisely and make them 'built-in' into the library part which is using them. This trick will reduce the amount of elements in your library and reduce loading time. You can read about the technique in section
5.2 Handling built-in images
of the documentation of the LP_XMLConverter tool (available from
http://www.graphisoft.com/support/developer/documentation/).
Regs,
Zsolt Táskai
ArchiCAD Development - GDL Team
AC13, AC14 and upwards...