Well... I cleared my picture cache and it made no difference whatsoever I'm afraid.
I thought the same as you Michael, that it pointed to a file location, but one thing I just noticed, when I started a new project using my "normal" template, which I had previously saved with some busses and those icons, that the icons are somewhat degraded right at the start. So those pictures may be part of the project perhaps...
I just tried an experiment... in prefs, I set the Track Icons location to a folder that does not contain any of those icon files, and the buss icons show up okay, so that confirms that the path location does not matter once the icon is loaded.
I then tried something else... I renamed the Track Icons folder, so that Sonar would not find it, and the icons still appear.
That surely suggests that the track and buss icons are somehow embedded into each project.
Having since cleared my picture cache, the only files that are in there at the moment are the ones associated with audio that I have only just recorded. There are five .WOV and five .TR2 files in there.
Hopefully someone from Cakewalk could shed some light on this...