1. An image alignment method, comprising steps of: receiving a first image and a second image; scaling the first image and the second image by a ratio to generate a first downsized image and a second downsized image respectively; determining a first offset between the first downsized image and the second downsized image; selecting a first saliency region and a second saliency region from the first downsized image and the second downsized image; determining a second offset between a first sub-region within the first image and a second sub-region within the second image, the first sub-region and the second sub-region corresponding to the first saliency region and the second saliency region respectively; determining a final offset according to the ratio, the first offset and the second offset; and aligning the first image and the second image by the final offset.