The xchat-gnome desktop uses "Icon=xchat-gnome" and xchat-gnome-common ships an /usr/share/icons/hicolor/48x48/apps/xchat-gnome.png, all the themes should fallback to hicolor and that works fine on my gutsy installation using the Human theme, to debug by somebody getting the issue
The xchat-gnome desktop uses "Icon=xchat-gnome" and xchat-gnome-common ships an /usr/share/ icons/hicolor/ 48x48/apps/ xchat-gnome. png, all the themes should fallback to hicolor and that works fine on my gutsy installation using the Human theme, to debug by somebody getting the issue