1. A method for automatically steering mosaic cut lines along preferred routes to form an output mosaic image, comprising the steps of: creating an assignment map corresponding to the output mosaic image where each pixel has an initial designation of unassigned; marking pixels of the assignment map that intersect the preferred routes as being Preferred Cut Line pixels to divide the Assignment Map into one or more regions that are bounded by Preferred Cut Line pixels or the edge of the Assignment Map; for each region, searching for one or more source images that completely cover that region, and responsive to multiple source images completely covering the region, using a Selection Heuristic to determine quality of coverage, and then designating each pixel in that region as being assigned to the source image determined to be best by the Selection Heuristic; for any remaining unassigned regions, searching for two or more source images whose combined footprint completely covers the region, and for each set of two or more combined images, using a Pairing Heuristic to determine quality of coverage, and then designating each pixel in the region as being assigned to the two or more combined images determined best by the Pairing Heuristic; redesignating the Preferred Cut Line pixels to match the image assignments of their bounded regions; and forming the output mosaic image by contributing pixel values from the source images based upon the designations set forth in the Assignment Map.