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

The header Image module allows you to display an image on selected pages via a block with flexible display conditions covering all pages of a site.

It doesn't support displaying images as a CSS background image, but here's how to easily achieve this.

At the time of writing the header image module doesn't support adding header images as a CSS background-image. Instead it simply displays the image field in a header image node. Using CSS background-images is more flexible in many use cases. Yes there are various background image modules but the advantage of using the header image module in this way is its ease of content management and full range of header image display conditions.

Entity background allows you to attach images and parallax images to entity types.

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

This a simple module that allows to set a vertical Parallax effect on the background of any element on the DOM.

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

This module provides a field formatter and Context integration for jQuery Backstretch - A simple jQuery plugin that allows you to add a dynamically-resized, slideshow-capable background image to any page or element.

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