This uses a SVG file I created of some pastel circles as a background image. This was inspired by a PNG file I created using an HTML5 canvas, but the PNG file was large, about a quarter MB. I converted it to JPG which cut the image size down to 40 KB, but lost transparency. This SVG file is about a kilobyte in size.
It appears that SVG files do not get treated the right way as background. CSS seems to not know the dimensions of the file and it only repeats the image vertically. Zooming the display doesn't solve the problem. I have tried the workaround of editing the image to make it color a moderate sized screen. The repeated image floats with the length of the body text. It is not fixed. Maybe using Javascript to randomly position the circles on whatever the canvas size is onload, is best.
The control of the image and its tiny size is very compelling as a way to make custom backgrounds. I am using Chrome 6 with a lightpink background-color and an SVG file which is transparent by default, so the background color appears where there are not colors assighed to the SVG objects. The Pastel Circles appear on a pink background.