I'm using FastStone Image Viewer for this, but I have tried XnView as well.
You can have up to four images and each fill almost a quarter of the screen. When you zoom in or out, all four images are zoomed by the same amount. When you zoom in so that the image is larger than its display area, you can click and drag on an image to pan it. As you pan, each photo is panned in unison, making it easy to compare the actual pixels in each image.