It depends on what distro you are using. For example, Mandrake the icons are in /usr/share/icons
and in SuSE they are in /opt/kde/icons. Your best bet is to open kde, open the kde find utility, and search for (for example) 48x48 under the / directory.
This will search for all files and directories named 48x48. It will point you to where your icons are located.