How Tiling Works

From: http://www.sci.kun.nl/thalia/guide/color/faq.html
Thalia: Guide: The background FAQ

How does `tiling' work?
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When you have specified a background image it always will be tiled.
That means the image will be repeated in the vertical and the
horizontal way. So, if you want a gradient color pattern on the
background just create a image which is just a few (say 8 or 10)
pixels high and very wide (so wide people will never see the tiling in
horizontal or vertical direction) and has the desired gradient color
pattern. The image will be very small (in file size) because it's only
a few pixels high. The browser will tile the image in the vertical or
the horizontal way so you have a perfect gradient background.

Here are a few pages that take advantage of background images that are
only a few pixels high (or wide): 

     http://www.inter.NL.net/ 
     http://www.sci.kun.nl/thalia/funpage/startrek/ 

Another `problem' with tiling is that your picture has to be
`seamless'. That means that your picture fits to itself perfectly.
Because when the picture will be tiled, the bottom of the picture will
be placed against the top of the picture. Also the right side will be
placed against the left side. Tom Karlo explains how to make such
seamless pictures with Photoshop on

http://the-tech.mit.edu/KPT/Makeback/makeback.html