Change these:
/usr/share/icons/crystalsvg/128x128/apps/kmenu.png
/usr/share/icons/crystalsvg/16x16/apps/kmenu.png
/usr/share/icons/crystalsvg/22x22/apps/kmenu.png
/usr/share/icons/crystalsvg/32x32/apps/kmenu.png
/usr/share/icons/crystalsvg/48x48/apps/kmenu.png
/usr/share/icons/crystalsvg/64x64/apps/kmenu.png
You may have to Alt-F2