I don't know if this effects other versions of GIMP as well, but I just ran into this with 1.3.11. I was poking around with my system and nuking packages (which is bound to make for a fun time :) ) and later when I tried to run GIMP I got a message about not being able to find "progressbar_trough.png" and the dang thing wouldn't run. After poking around for a while I discovered that the problem was that I didn't have x11-themes/gtk-engines-eazel installed, although this was not listed as a dependency of GIMP. Upon installing the package everything's hunky dorey.
ACtually thats probably because you had the gtk+ theme listed as Eazel for either gtk 1 or 2.. they dont take it that well when you remove the theme engine from beneath their hands... theres no explicit dependency on the engine though.
