Background images are those image files which appear literally in the background of this and every page on this wiki. Many wikis regularly use -- and change -- such images. However, we abandoned the use of background images at around the time of the broadcast of A Christmas Carol.
In most page views, background images are not seen on Tardis. The main reason is because of the rise in tablet and phone usage -- where background images never show -- as well as page views containing background advertisements. Most people simply never see the background, so we've adopted a cleaner background look.
Therefore, it's the general design policy of the wiki not to have a background.