@Louis, I did search for related information on this as I know it had been discussed in the past but was unable to find anything conclusive, so I raised it.
We have changed this to a feature request as it seems more applicable.
We have a very image heavy upcoming website with background images that change based on the responsive breakpoints, so this will be a a very very timely addition, and will save us a lot of time, even with the dynamic aspect still pending.
I appreciate there are usually considerations when adding features and I am grateful for you choosing to add this.
Improvements are always being considered while we develop bigger features.
We are aware and have planned for the improvement and rework of the background image properties.