A method including dividing a first image into first sub-images; calculating a first checksum value for each of the first sub-images; dividing a second image into second sub-images; calculating a second checksum value for each of the second sub-images; comparing the first checksum values with the second checksum values; and determining whether one or more differences in checksum values exist between the first checksum values and the second checksum values and correspondingly whether one or more differences exist between the first sub-images and the second sub-images.