title: How to get difference between 2 images and save it to an image
body: <p>And what i mean by the title is 
  I have 2 images - each one is from a different time ( from screen cap)
  i want to get the difference between the two 
  make all the same parts transparent,
  send the difference to a client
  and place it on top of an image that he has</p>
  
  <p>Don't worry about the sending part - I have covered that.
  The thing I need help is how to get the difference, save it to an image / stream / byte array
  and merging it to one image.</p>