You can use Gtk. Although I'm unfamiliar with Tk, I've used Gtk; and, as I understand it, Tk is easier to work with and produces good results.
If, however, you become interested in Gtk, this is a good resource.
There's more information about GTK at http://www.gtk.org. Note that the big drawback to using GTK is that there's no stable Win32 implementation yet,
although work's being done on in over at http://user.sgic.fi/~tml/gimp/win32/.