This plugin is a WordPress core feature-plugin that adds cropping to background images for parity with header images. For this particular feature the implementation is quite simple, but it’s starting out as a plugin to gauge user interest in this feature and whether the feature improves the user experience of background images.
This plugin requires WordPress 4.3 Beta 3 or later.
If you’re interested in contributing to this project, ping @celloexpressions on WordPress core Slack. Please comment on the trac ticket with ideas and feedback: https://core.trac.wordpress.org/ticket/32403.