For banner images to have the parallax effect enabled for mobile devices, access the Layout portion of the Theme settings to activate the Enable parallax on mobile setting.
Please note that parallax-enabled images will always be 'cropped' on mobile.
Cropping 'zooms' in on the image and typically allows for a greater display area on mobile instead of a narrow/full-banner. The height of the banner then gets defined by the text. The more text inside of the banner, the larger the image becomes center-cropped.
Full Banner Images on Mobile
If you do not enable the parallax effect on mobile for banner images, you can crop them on mobile as well, or choose to display full banner images.
To crop non-parallax banners on mobile, access the Layout portion of the Theme settings to enable the Crop non-parallax banner images on mobile setting.
If you do not enable that setting, the height of the banner image will be defined by the image's height. This will ensure the image is not cropped, and if the amount of text exceeds the height of the image, the text will be cropped.