Mark center of map?

[Richard A. Milewski]

Can someone point me at a how-to that describes how to draw on a map?  What
I need to do is plant a big red X at the very center of a map served by
mapserver.  The Symbology reference seems to be all about how to place
symbols at locations defined in a dbf file, but I need to plot this symbol
at the center of whatever extent the user has chosen. The maps are specified
by Lat/Lng, so I know the coordinates but not until they request is actually
sent to MapServer.  Running MapServer 4.4.1 on Linux  (but not Mapscript
...yet).

I can build a script that fetches the image and then places the symbol in
the center of the image with netpbm or image magick, but that seems crude,
and I suspect there is a way to do it inside mapserver, but I haven't found
it yet.

Thanks for the help!

-- Richard