I can't tell if you got this working or not. I'm not exactly sure how all this stuff works, but here's something that always seems to work for me:

Create a 48x48 .png file and call it killerapp.png. Put it in:

/usr/share/pixmaps

Inside:

/usr/share/applications/hildon/killerapp.desktop

add the line

Icon: killerapp

I'm not sure, but you might have to reboot to see the icon in the Application Manager and/or on the Desktop. If you're not seeing it on the Desktop, you might have to delete and readd the shortcut.