Identical for images is a really difficult thing to figure out. If you take an image, scale it up, then scale it back down to the original size, the result might not even be the same pixel set as the original. Every image manipulation program (Photoshop, GIMP, OpenGL) likely uses a slightly different algorithm to scale images. For linear transformations, it should always be the same, but most programs will do interpolation, not only will each program do it differently, there are many different algorithms (bilinear, bicubic, all can be affected by the amount of bleeding and intensity of bleeding falloff.)

There are some really nice comparators out there though, google has one, so I'm sure you can find one on their repository. OpenCV also has one that I've used before.