# Add this code before \begin{document} 

\usepackage{eso-pic}
\newcommand\BackgroundPic{
\put(0,0){
\parbox[b][\paperheight]{\paperwidth}{%
\vfill
\centering
\includegraphics[width=\paperwidth,height=\paperheight,
keepaspectratio]{background.png}%
\vfill
}}}

# and this immediately after \begin{document} :

\AddToShipoutPicture{\BackgroundPic}

http://kuscsik.blogspot.com/2008/03/how-to-background-image-in-latex.html

Background image in LaTeX

image

LaTeX