I use the Faenza icon collection in Ubuntu 12.04 Unity with no issues. I decided to change my Gimp launcher icon, so I made my own (gimp-ak.png) and added it, and its appropriately sized derivatives, to the Faenza icon folders:

/usr/share/icons/Faenza/apps/16/gimp-ak.png
/usr/share/icons/Faenza/apps/22/gimp-ak.png
/usr/share/icons/Faenza/apps/24/gimp-ak.png
/usr/share/icons/Faenza/apps/32/gimp-ak.png
/usr/share/icons/Faenza/apps/48/gimp-ak.png
/usr/share/icons/Faenza/apps/64/gimp-ak.png
/usr/share/icons/Faenza/apps/96/gimp-ak.png
/usr/share/icons/Faenza/apps/scalable/gimp-ak.svg

I then updated the Icon field in /usr/share/applications/gimp.desktop from "gimp" to "gimp-ak":

[Desktop Entry]
Version=1.0
Type=Application
Name=GIMP Image Editor
GenericName=Image Editor
Comment=Create images and edit photographs
Exec=gimp-2.6 %U
TryExec=gimp-2.6
Icon=gimp-ak
Terminal=false
Categories=Graphics;2DGraphics;RasterGraphics;GTK;
X-GNOME-Bugzilla-Bugzilla=GNOME
X-GNOME-Bugzilla-Product=GIMP
X-GNOME-Bugzilla-Component=General
X-GNOME-Bugzilla-Version=2.6.12
X-GNOME-Bugzilla-OtherBinaries=gimp-2.6
StartupNotify=true
MimeType=application/postscript;application/pdf;image/bmp;image/g3fax;image/gif;image/x-$ X-Ubuntu-Gettext-Domain=gimp20

After logging off (and even restarting), my custom icon does not show up - Gimp has the default gear icon:

Setting the Icon field in gimp.desktop to any other icon in the Faenza collection works fine. What do I need to do to get my custom icon to show up properly?