How use a image as background for a web page

Hello all
I would like to use a picture as background and on the pictures insert a table
I'm trying with BackImageUrl,  but I don't know exactly how insert it, to use for the
<asp:Content runat="server" ContentPlaceHolderID="Contenido1"> or Do I have to insert it for the table?
Thanks for the help!
 
 
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