On my system the image lives at $KDEROOT/share/apps/kicker/pics/kside.png where $KDEROOT is the top-level directory of your kde installation. It could take years to find which config file specifies this image, indeed it may be hard-coded into the binary...so I suggest backing up this image and dropping your own in with the same name.
Show us a screenie of what you come up with
?
