As you now, Slackware Project does not prefer changing any components of GNU/Linux OS, desktop environment or desktop manager. It is aimed that stability, simplicity and being most UNIX-like distribution. Software packages in Slackware come with a few modifications as much as possible. As a result of this, desktop environments run on Slackware are lack of visual components special to Slackware unlike other popular GNU/Linux Distributions such as "Debian", "Red Hat Enterprise Linux" or "Suse Linux Enterprise". I have redesigned a new KMenu Icon for Slackware using the original KDE logo at Wikipedia.
To change KMenu Icon in Slackware 13.37, simply right click on the KMenu Icon on KDE Panel, then follow below path.
"Application Launcher Settings > General > Icon > Other Icons > Browse > Downloaded KSlackMenu Icon (PNG) > OK > OK"