I've been playing around a bit with the various ways to invert images using AutoIt. A search of the forums has developed several good methods, some faster than others, many using various specialized modules to get the method to work.
I've been playing around with Malkey's script from this thread, and it's fast, but I've noticed it tends to drop information from images.
For example, here's the test image I used:
Here's Malkey's script I used: