I've been looking for a solution and haven't found an easy way just yet. This feature exists in core, but is only available when in the customizer. Think theme options header images etc. I've seem some posts (that are aging) and this stackexchange thread with some nice solutions.
Just thought I would drop a line, don't know if others would be looking for this or if it's easily build-able into the core of the image fields in MB.
This is a nice improvement. I've just looked at the solution on SE and I think it's quite complicated. Working with WP's Media Library is not easy! Our code for the media fields are not simple, either.
I'll make a note on this and see if I can improve it. Thanks for your suggestion.