The shifting could be done by inserting a bit of CSS inline to set the background-position property. Shouldn't be too hard. The hard part of that would be to design the interface and get the zooming options.
The idea is nice, from a user's perspective and I can imagine a handful actually using it. But tend to doubt that it is worth the effort. Moving the picture is probably easy, with a little bit of javascript. But then zooming already requires something that is almost a graphics editor, since it must stretch the image visually, provide the interface and even sharpen it and save it to disk, you get the picture. Just use one of the many editors out there. The good part is: that also works on any other forum or whatever you need a picture for. ;)