Hello,
This is the way WP Bakery Page builder works by default. You have two ways: 1. hide the column on mobile and create another row with columns visible only on mobile. In the first row (with column hidden on mobile you set video) on the second row (column hidden on all the devices accept mobile) you set the image.
Here is the screen how to set the visibility of the column on different devices:
1. http://prntscr.com/j4fho7
2. http://prntscr.com/j4fio2
Way 2: you can assign class or ID to row and write image background on mobile in custom CSS.
If you need custom CSS please provide your site admin access and image url
Best Regards