cc8a9d26_S_background_images___W3C_Wiki__Property
[Definition] Tells the browser where to position the background image. [Description] Images can be displayed anywhere within the borders of the HTML element on which they are applied. Use background-position to precisely place your images for visual effect and layering. There are many useful ways to indicate background position, keywords and numeric values. Keywords (such as top and bottom) are very useful and easy to read. Pixel values are very precise, but don’t adapt to changing heights and widths. Negative pixel values are useful when using CSS sprites, as you’ll find out later. When percentages and pixels are used, the starting point is always the top left corner of the HTML element, although the way image positioning works with pixels and percentages is rather different. Pixels always move the image a set number of pixels towards the bottom and right of the containing box (or towards the top and left if they are negative values), regardless of the size of the image and the containing box. Percentages on the other hand move the image a percentage of the difference between the containing box size, and the image size. If the image and the containing box are the same size, percentages won’t move the image at all. Valid values include length (generally in pixels), percentage (of the width of the element), and the keywords top, right, bottom, left, and center. Note that center can be used to indicate both vertical and horizontal center. Note also that you can mix percentages and pixels in rules, but not keywords and pixels or keywords and percentages. [Property] 
position
b a c k g r o u n d - c o l o r
i m a g e
r e p e a t
a t t a c h m e n t
p o s i t i o n
b a c k g r o u n d
CSS background images - W3C Wiki