This would depend on whether you knew what the algorithm used on the original image was. If you did, you might be able to make an estimate of the original image. But if you only have an altered image it would be very hard.
For comparison, imagine you put a picture in paint and inverted the colour of half the pixels. This is easy to undo because you can easily guess the process that changed the picture.
On the other hand, if you darkened all the pixels until they were black this would be very hard to reverse unless you knew exactly how much each picture was changed by. This is because the final image has lost a lot of the information the original picture had.