Icons are found in 2 places:
Icons that were installed by the administrator/root are found in /usr/share/icons
Icons that were added by a regular user is found in their own home directory, in ~/.kde/share/icons
Icons are grouped by theme/name, then by size (16x16, 48x48, etc), then by type (application icons, mimetype icons, filesystem icons, etc). You can easily replace individual icons with another icon. The hard part is figuring out where to put that icon in the first palce.