Blimey! Well, I'm very flattered — thanks — but
also a little embarrassed, because on this particular subject I'm
on shaky ground. I had thought that the TOC icons were stored in
the HTML Help executable (%windir%\hh.exe), but Steve Wexler's book
"Official Microsoft HTML Help Authoring Kit" says that they are in
the HTML Help ActiveX control (%windir%\system32\hhctrl.ocx)
— which makes sense when you think about it, because the TOC
tree view in the navigation pane is an instance of that control.
Both hh.exe and hhctrl.ocx are protected system components,
so I doubt that anyone wanting to hack them would get very far. On
the other hand, in the relatively early days of HTML Help, some of
the Microsoft MVPs did find a way to change the TOC icons
programmatically. In a newsgroup posting, Dave Liske refers to
"tracking down the handle of the treeview control and changing the
image list via the Win32 API. It's definitely not an easy task but
it can be done." But that was eight years ago, before the advent of
Windows File Protection and all the additional security features
that now surround HTML Help.
Pete