When to use CSS background-image
Use CSS background images if the image is not part of the content. —sanchothefat
Use CSS background images when doing image-replacement of text eg. paragraphs/headers. —sanchothefat
Use background-image if you intend to have people print your page and you do not want the image to be included by default. —JayTee
Use background-image if you need to improve download times, as with CSS sprites.
Use background-image if you need for only a portion of the image to be visible, as with CSS sprites.
Use background-image with background-size:cover in order to stretch a background image to fill its entire window.