(These last two appear to switch places!)
I think this pair worked the best because 1) the images are so similar and 2) the fuzzy outlines of both subjects obscures the differences in shape. Experimenting with filter sizes, I decided the the best results for this image came when the frequency ranges for each image overlapped slightly; when the lion was blurred slightly less than the image subtracted from the cat. Here is each image along with its fft.
Origonal lion image:
Lion convolved with a 40 pixel gaussian:
Original cat image:
Cat, minus itself convolved with a 50 pixel gaussian:
Sum of modified images:
I think this pair was the least successful because the large, bright, airplane wing in the middle of the first image conflicts with the dark band of space in the middle of the second. I tried rotating them differently when aligning them, but since the conflicting areas were right in the middle, this didn't help. The outsides of each image, however, match up very well.