Officially, Excel does not support background printing. The best way we found is to use header and footer. View->Header and Footer..., then we can insert a picture as a background. Don't worry, if the header is a picture, it will be lying rather under the table than stack on the top of the table.
One more thing, is that normally, a picture file is obvious large compare with a excel file. So if someone just need a text based watermark, then wmf file is not a bad idea.
