I used GIMP
, a free image-manipulation program. Photoshop would probably work as well.
The image was transformed by deleting the blue-gray background with the area-select tool (set somewhere between 30 and 60, I don't recall), then filling in a few areas of lost detail (which happened to be too close in color to the background) by hand and deleting a few stray pixels the same way.
Too keep transparency, the image was saved as a gif (which can handle transparency), rather than a jpeg (which can't).