Head to ~/.kde/share/icons/ This is where your user installed icon themes will be (as long as they were installed using the tar.gz files like from kde-look)
Select the folder of the icon theme that you are using, or that you want to use.
Go into the size folders (16x16, 32x32, etc) and into the apps folder. The 2 most common are 32x32 and 48x48
Locate the icon named kmenu.png and rename it to menuk-mdk.png
Then reload your icon theme (K menu -> System -> Configuration -> KDE -> LookNFeel -> Icons, select a new theme, then your original theme and click apply)