Tcl/Tk is a language for building graphical user-interfaces under the X window system, both free-standing graphical programs (e.g. image displayers) and interfaces to packages. While not perfect, it is a major improvement on Motif.
The standard reference for Tcl/Tk is the book by Ousterhout. The language has been extended somewhat since this book. In particular, features have been added to handle digitized images, mysteriously missing from the original specification. For these, see the on-line manual pages that came with your system's copy of Tcl/Tk.
See also the on-line manual and Sun's TCL page.