http://www.processing.org/ is a web site dedicated to "an open source programming language and environment for people who want to program images, animation, and sound."
Appears to be available for GNU/Linux, Mac OS X, and Windows.
See also http://dev.processing.org/ for developers, http://hardware.processing.org/ , and http://mobile.processing.org/ .
Again, I don't see a single reference to Tcl/Tk on this web site - but we may find useful ideas that could be added to Tcl and Tk.