Invert image by using a curves adjustment layer:
Move the corner points to the opposite corners.
Can use a master change on the RGB and/or make
changes on each color channel.

Make a second adjustment layer (curves or levels)
to adjust the contrast.

Then paint on the layer mask to reveal portions
of the original image.

forum discussion link: http://forums.dpreview.com/forums/read.asp?forum=1006&message=20990938