Image Magick III - Invert an image

Three ways to invert an image:

convert black-white.png -negate white-black.png

convert upside-down.png -rotate 180 rightside-up.png

convert upside-down.png -flip rightside-up.png

From: http://www.imagemagick.org/pipermail/magick-users/2003-October/011071.html