remove equal pixels

Hi!
Can i compare two images/layers pixel by pixel and remove equal pixels?

For example: I have two pictures of the same wall. One of the picture has a fly on it. So i need to remove the wall and left just fly.

it's like in .gif format. the same pixels in frames are removed.

....or should i post to Scripting community?

I would try something along the following lines: Convert the two images to RGB mode an place on separate layers. Set the top layer to Difference blending mode. Stamp to a new layer. Use a threshold adjustment with the threshold cranked down to where all the identical pixels from the two images are black and all the others are white (this will probably be almost all the way to the left. Stamp this to a new layer, and use it as a mask.