I think I understand what you are saying now.
Without sounding lazy, I was hoping there would be a way to do this where the dynamic slide randomly picks an image to use for its background from a selection of 3 or 4 images. And hopefully this would not alter my posts or alter how I post.
Is there a way around this without me having to alter the way I make my posts? If the best or easiest option is to simply select a custom field (given by ACF) then that is fine, I will give it a shot. ??