Delving into the codes of other sites, not once met the pictures, which combined background images for different elements (for example, colored buttons). For example, this is how this http://www.webmolot.com/i/menu_sprite.png
Can you tell me how this is achieved? You can link to any article about this, or search query =)
Addition: under the word "this" I mean styles that indicate exactly which part of the picture to use for a particular element.