Scott RegiStar has a 'hidden' function where if you align two or more images you can then combine them using intersection and if you set the original image weight to say 0.0001 you then get an identical sized and oriented image of the second. This is very handy for making animated gifs and flicker seaching for changes.
Here is an example
http://members.optusnet.com.au/~trlee8/ComLovejoy.gif
Bert