OTish Overlaying images

Hi All, Does anyone know of an application which would let me define reference points in two (or more) images such that the two can be overlaid on each other. Specifically, I am hoping to lay images of maps over aerial photos and other maps, so the software would need to be able to calculate the necessary rotation and re-scaling to fit an overlaid image to a base image. Thanks for any suggestions. Dylan -- "I see your Schwartz is as big as mine"                                   -Dark Helmet