Replacing a window icon in X11?

[Wirawan Purwanto]

Hi,

Today most window managers has an area on the top left corner of the 
window decoration which shows a small bitmap, which in Windows world 
called "icon". I wonder if there is any way for an external program, if it 
knows the window ID of a given X11 window, to change that image to 
something else. Older X programs such as xterm seems to not have an option 
to set this icon pixmap (or did I miss it?). If I have a lot of such 
programs open, then the "task bar" of desktop managers (xfce/gnome/kde) 
only shows a bland box or "X" icon, which is not meaningful. I wonder if I 
can replace that with a more suggestive pixmap, thus speeding up my search 
for a particular window.

I am quite uneducated about X11 programming, so if you can give me the 
subroutine name, header file, and pointer to more information, I'd 
appreciate it. If it can be accomplished using gtk, I'm fine, too.

Thanks,
Wirawan