1. A method of aligning a pair of original images, the method comprising: defining a plurality of different pairs of images at a plurality of different resolutions, wherein the plurality of different pairs of images is associated with a plurality of different pairs of bitmaps, and each particular bitmap is a bitmap of a particular original image at a particular resolution; and hierarchically aligning the pair of original images by first aligning the pair of images at the lowest resolution and then aligning each pair of images at each higher resolution based on the alignments of the pair of images at the lower resolutions, the hierarchical aligning comprising, for each particular resolution in a subset of resolutions: for at least one particular image at the particular resolution, dividing the particular image into a plurality of tiles for use in identifying corresponding portions in the other image in the pair at the particular resolution; and comparing the pair of images by using tiles that are suitable for the comparison while discarding tiles that are unsuitable from the comparison; wherein identifying suitable and unsuitable tiles in a particular image at a particular resolution comprises identifying tiles that have more than a threshold level of black pixels or tiles that have more than a threshold level of white pixels as unsuitable.