I could just get the same effect by changing the FOV in the console and then be able to see anything above my head in the direction I'm facing, which is a lot more useful than just a top-down perspective and it doesn't really make a useful radar. I suppose you could make a cool dynamic map GUI with it. It'd be totally client sided too.
I think you are better off just skipping the image. I mean seriously, how many pictures of radar have you seen in movies include the radar image layered over a satellite image tracking the boat? At best you might get a GPS map with terrain contours on it, or something like a GPS in a car with a road overlay. And this isn't radar it's GPS.
I haven't played it because I have no interest in the server but Diggy's Metropolis RPG seems to already have radar in it.
So make a working radar first. We actually need a public radar mod for this to go in, unless you just plan on handing your code off to Diggy and having it disappear into the black hole of private mods. Worry about the pictures later. If you can't find a way to make the image then one method which should work is that you could just provide documentation for mappers to produce a radar.png to put in the map folder of every map they release. Then the radar mod could just pull it out of the map folder, and if it didn't exist, it could fall back to a static white noise tv effect instead. Include radar maps for the default maps with the mod. Slate style maps could just use the bottom sky or water textures. The server mod should be able to pick through a map and figure out if it's a slate map (has slate.dif in it) and then pick the water texture first, and if there is none, fall back to the sky texture.