App icon doesn't show in Unity Dock
 

Hi:

Yesterday, I installed the RPN calculator (grpn). When I dragged it to the Dock it didn't show any icon (except the default question mark icon). Ok, I removed it from the dock, went to the applications folder, edited the desktop file, and assigned one of the icons from the 64 pix folder. Then, I dragged the desktop file from the applications folder to the dock, and viola, I now had the app with an icon in the dock. The problem is that after I restart the pc, the dock shows the app with the question mark icon again, instead of the one I assigned to it. In the applications folder, the desktop file shows correctly the icon I selected. Tried the previous exercise a couple of times with the same result. Am I missing a critical step to make the dock recognize the icon I selected for the app? Your help is appreciated.