In version 9 we can set the background to an image, however I'd like to save this into a stylesheet that can be installed.
sample image you want to use as background:
image = Import["ExampleData/lena.tif"];
Create cell that you need to put in style definition:
Cell[StyleData["Notebook"], System`BackgroundAppearance -> image] // CellPrint
Open stylesheet editor (Format -> Edit StyleSheet...)
Copy the cell you just created and install stylesheet.