drawing on a background image

[christophe Meyer <chr meyer nerim net>]

Hi,
In order to write weather satellite tracking software (using glade) i have to 
draw satellites footprints on a background earth-image such in this windows 
example:
http://www.david-taylor.pwp.blueyonder.co.uk/software/wxtrack_at_work.htm
Does anyone know how to draw on a background image ? Has anyone
already done something like that ? Has anyone an example of such an 
application ? How can i plot a simple pixel on an existing image ?
thanks for your help :)

Chris

ps: there is an example in Evolution (on the timezone map) but it is
a bit complicated and i don't understand it :(