Actually, you only need to delete the after image to import a before image of a different size.
The error you get actually refers to the fact that the before and after images are different sizes therefore if you delete either image and import a new image for the one that is left it will resize the button for the new image. Then when you import an image for the one that is empty it must be the same size as the other.
There are 10 types of people in this world, There are those who understand binary, and those who don't!
