As of today, clients can make images 50% width and then have two images on one plane. But its impossible to make that same images 100% on mobile. And the client also have to make sure the images is the exact same size.
This site has a good example.
I think this had to be done with a image blocks. Where you can choose a layout of the images. That solves the problem if the images is different height and width*. And the client don't have to think about mobile.
* The image block will work as a div where the images is the background. So images that is different sizes will be cropped.