ImageMagick display, 7 18 24 bytes

-negate

or

-fx 1-u

The ImageMagick tool display can apply the above "programs" to a png given as the parameter and display the results on screen.

Check out my post on meta about ImageMagick as a programming language. I wrote a prime tester over there as a proof of concept. I am not yet clear on whether or not it is legitimate to omit the "-fx " and get this score down to 3 bytes