How to find rectangle of difference between two images

A naive approach would be to start at the origin, and work line by line, column by column. Compare each pixel, keeping note of the topmost, leftmost, rightmost, and bottommost, from which you can calculate your rectangle. There will be cases where this single pass approach would be faster (i.e. where there is a very small differing area)

Image processing like this is expensive, there are a lot of bits to look at. In real applications, you almost always need to filter the image to get rid of artifacts induced by imperfect image captures.

A common library used for this kind of bit whacking is OpenCV, it takes advantage of dedicated CPU instructions available to make this fast. There are several .NET wrappers available for it, Emgu is one of them.

I don't think there is an easier way.

In fact doing this will just be a (very) few lines of code, so unless you find a library that does that for you directly you won't find a shorter way.