I am working with MB Builder and Beaver Builder Themer. When I try to connect to a custom image field it will not show anything on the page. Text and Number fields will show but not a single image. I will show up in the edit post screen but not final post.
When you choose a field to connect to (like a heading field), you'll see Meta Box Field under the Posts group. And from there you can choose which field to insert. Please try and let me know how it goes.