where * can = 16x16, 32x32, 48x48, 64x64, 128x128
#you need to be root to proceed so
#if in your menu you have an option like run file-manager as root by all means click that otherwise in terminal:
gksu pcmanfm "/usr/share/applications/"
find > amarok.desktop
now edit the file > gedit amarok.desktop
#if you do not have gedit use what ever text editor you have available
#just make sure you launch the editor from within pcmanfm so to attain root privileges.
#change the icon reference to reflect one that exists on your system
#save and close.