Is the problem that your image gets cropped? This is something you would need to rework in the code. As it is now, it's a background image, the image doesn't get it's height and width from the image itself and instead is set with CSS. The way I would do this is to rework article.liquid to use an <img> tag to set the height of it's container instead of a background image for the featured image, and then swap out that image with a different mobile image, who's location I would store in a metafield.
I don't expect you to know what I'm talking about, but that is just how I would approach it. I would suggest not using any text in your image, and making sure the focal point of your image is in the center if you're not comfortable with making edits to theme code.