The problem is similar to "barrel distortions" of pixels at the edges. That is, they become disproportionatly distorted compared to the center. The same problem is similar in panoramic images. There are a few 2D morphers that can apply a corrective distortion but the learning curve & trial & error may not be worth your while. Look around the links at the PTStitcher page to find some great free apps & more info: http://www.fh-furtwangen.de/~dersch/