HAha.. I made a program some time ago that was a Spot the Difference game.
I made two images, changed one, and had both on the screen. One above the other.
What might help, is I used the set current bitmap number to have two images on the screen i.e. I made a seperate image with coloured circles where the image was altered, made the rest black. When the mouse was clicked, it checked the second image to see what the colour was. That meant I could have 5 different changes.
It was a pain to record what number a colour was (using get pixel) I think. Blue is 5305938 or something bizzare.
You never saw the coloured spots, as this was on a different screen to the images. It's like flipping a back buffer screen.
bleep bleep