Located at Mawson station there is a webcam that gives a picture of Horseshoe Harbour to the outside world. The camera is actually a live-feed video, but due to the costs associated with satellite internet access only still pictures are available on the internet.
You can locate the Australian Antarctic Division's webcam page for Mawson Station here. Note that video from this live feed cam is converted into a webcam image once per hour and as such you'll need to time your cache find webcam picture with the time the camera takes the webcam picture (exact picture times are available from the station when you are there). Since the picture is taken about every 10 minutes, you can pose for the picture while holding your GPS unit and then visit one of the communal computers available on the station to download your picture and then upload it with your cache find.
The camera itself is located on the yellow building and points in the general direction of the fuel tank farm that is not labelled "Mawson". Note that the camera does not operate after darkness falls.
For a valid find, you physically must visit this Antarctic base -- No cut-and-paste photos of yourself on top of the webcam image!