Is it possible to change the image only when on mobile?

Yes, you can!

For example, if you want to change the main visual only for mobile;

1. You can add an image that you want to display on mobile by adding Overlay on top of the main visual image in default mode
2. Select the image and place it in the upper left corner of the editor In some view settings, if you set ✓ on mobile only, only the mobile image will be shown.