The Google Maps API can be used for other things than maps.
The tiles were created with this python script: pngmaptiler-20100427. It will convert a PNG image to PNG tiles. The script depends on pycairo to do the cutting and scaling of the images. This example converts master.png to tiles of size 256×256 with 1 being the minimum zoom level and 4 being the maximum zoom:
$ pngmaptiler.py master.png 256 256 1 4