How to find the differences between two colored images?

Dear all, I have original image and another one that has something missing. I need an algorithm to find this missing thing between these two images and make circle or square on the place of this missing thing. Could you please help me?

Thanks for you all

Answer 1

[Image Analyst]

Are they the same size? Why not simply subtract them?

diffImage = double(image1) - double(image2);