AFAIK, there are two options:
1) Change the source & recompile.
2) change the actual icon in /usr/share/icons (+ /crystalsvg maybe?). So keep the name the same, but change the icon.
The same thing goes for the "show desktop" icon. The icon is called desktop-mdk.png, and is somewhere in either /usr/share/icons or /usr/share/icons/crystalsvg; forgot which place exactly.