All modern browsers can display background images.
These can be small rectangular images that the browser 'tiles' to fill the window. The background of this page, for instance, is a small GIF file that repeats to fill the space. Remember that type will usually have to go over your background image. If the background has too strong a pattern, it will make the type hard to read. You can put dark type on a light background and light type on a dark background without too much trouble.