Some strange behaviour of before_after when the images are smaller than the surrounding container. The before image will be attached to the left edge and the after image to the right. So if they are not at least the size of the container they don't overlap properly.
In the start page provided with the theme you use a width and height parameter in the shortcode but the php code behind it doesn't do anything with it and the shortcode generator also doesn't know width and height as parameters. Seems like this was an intended functionality which didn't make it into the product.
Any plans to fix this and to introduce width and height parameters? Otherwise it is quite useless (or the solution might be to surround before_after by a fixed width div).