The Simple background-image formatter module allows image fields to be formatted as a background image on a html tag.

https://www.drupal.org/project/simple_background_image_formatter

This module provides an image formatter that allows you to display the image in a div as background image. The module extends Drupal's images field.

https://www.drupal.org/project/background_image_formatter

If your site is designed around utilizing background images, then this module is for you! Whether you need a surgical implementation that only administrators/developers can implement or provide the ability to allow users to attach their own background images to entities, this module has you covered.

https://www.drupal.org/project/background_image