The page background here is actually
just one very short, very wide image.
In HTML, when a background image is specified in the body tag, the image is then automatically repeated endlessly, both horizontally and vertically. This is called, appropriately, tiling.
The image with the black border below is the same image that is being used for the background of this page. If you can scroll far enough to the right you will see the background repeating again. Most viewers either can't or won't bother to look that far, so they never notice the image repeating to the right.
In addition, the image that is used is prepared in a manner so that its bottom edge will blend in seamlessly with its own top edge. That's why you can't see that the file is actually repeating vertically.
This is a standard practice for creating backgrounds on the Web. It's popular because the image file size can be very small and yet appear to be large.
The background image for this page
consists of the above short and wide
image repeated endlessly.
Scroll to the right.
You'll see where the background image starts
to repeat itself.