The repeat top left are for positioning. You could also use other combos. For instance if you wanted the image 50px in from the left and 100px down from the top and wanted it to not repeat all over the screen and stay fixed you would use.
HTML Code:
background: url(/testing/wp-content/themes/atahualpa/images/backgr.gif) no-repeat 50px 100px fixed;
If you go to the
Tryit sections of the W3 Schools site you try the code out and see what it does.