I managed to injure my system. I was attempting to change the theme Nautilus uses.
I was Googling for ideas, and I guess one of the suggestions I tried was less than optimal for my configuration. Unfortunately, I don't know which suggestion it was.
Not only was I unsuccessful, now the GTK apps I use display a default icon (a red x) both on the button in the top left corner of the application window and in the task bar button for the app.
The affected applications I've found so far are Nautilus, GEdit, Gnome Terminal and Eye of Gnome Image Viewer. Applications such as Geany, Thunderbird, Firefox and VLC show their normal icons.
I am using IceWM 1.3.7 on Ubuntu 12-04. When I switch to XFCE, the GTK apps show their normal icons. Then I switch back to IceWM and they are back to showing the default icon. On my backup computer, the GTK apps show their normal icons both under IceWM and XFCE.
How can I get the GTK apps to display their normal icons?