Icons in the menu do not change when the theme is changed. This
is true even if the theme has an icon of the correct name.

The cause of this is that in
/usr/share/applications/gimp-2.2.desktop
the Icon= line has the absolute path to the icon instead of just the
icon name.

Icon=/usr/share/gimp/2.0/images/wilber-icon.png
When it should have:
Icon=wilber-icon.png

The icon should be put in the hicolor theme, per freedesktop.org.
/usr/share/icons/hicolor/48x48/apps/

See also: https://bugzilla.redhat.com/bugzilla/show_bug.cgi?id=119991