We use a great many image comparisons in our testing, these work well however the use of masks is far from intuitive and is cumbersome.
May I suggest that masks can be created interactively with the target image, maybe allowing the user to paint an area, or areas, to exclude.
Furthermore, when the images are compared following a test then the ares of the image that are excluded should be displayed.
Thanks Ray
This feature is common in other tools