From The Hive:
DiffImg is a simple image comparison tool which takes two images with the same size as input. Some statistics are computed and the positions where pixel differs are displayed as a color mask. It is used to check compression level comparison, raytracing parameterization optimization, and non regression tests for 3D engine. DifffImg use QImage as inner format so it has been tested only on RGB image, so alpha channel or float texture aren't take into account. Features include image difference display: a yellow or red mask can overlay the original or modified images, single or dual panel mode, statistic computation: mean error, RMS error, and min/max are computed, histogram of error, "Slideshow": can compare directory of image so you can switch to the previous or next comparison if you have two folders with the same image list, it can read basic formats as BMP, PNG, and JPG, you can add more formats if needed by using bridge plug-ins to the FreeImage or the Devil libraries, and basic batch mode.