Importing a GIF image and making it transparent

Hi all I've been trying to make a script that imports a GIF image from an external server (i have permission to do this, see below) and designates a certain colour transparent for some time now and i'm not having much luck. Can anyone help or provide a script that does this? The images i'm using are Australian weather radar images with a rather plain brown background http://mirror.bom.gov.au/radar/IDR503.gif What i'd like to do is have an orographic map for the background, like this http://www.bsch.au.com/temp/preview.jpg The radar images are available to the public and i have permission to make these alterations Is it possible for PHP to download a GIF image (through HTTP), designate a certain colour transparent and then display the image on the screen without saving the image to the server? Any help i can get with this would be much appreciated