Here you go:
Put this in the Notes field:
<img src="http://path.to.image/source" style="position: fixed; top: 50%; left: 50%; margin-top: -250px; margin-left: -250px; opacity: 0.5; z-index: -99;">
Replace the margin-top value with negative one-half of the image’s height, and replace the margin-left value with negative one-half of the image’s width (the example above is for a 500x500-pixel image). You can tweak the opacity value from 0 to 1 to get the look you want. The z-index is key, as it specifies that the image should appear behind everything else. Note that when you preview the invoice on your screen, you won’t see the watermark, but it will appear in the output when it’s printed.
Result:
