transparency of background image

Question

How to set the transparency of the background image of the frame in beamer? I have tried this.

\documentclass[bigger]{beamer}
\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc}

\usepackage{graphicx}


%Global Background must be put in preamble
\usebackgroundtemplate%
{%
    \includegraphics[width=\paperwidth,height=\paperheight]{Doc8}%
}




\begin{document}

\begin{frame}{Introduction}
\begin{itemize}
\item 1
\item 2
\item 3
\end{itemize}
\end{frame}


% Local background must be enclosed by curly braces for grouping.
{
\usebackgroundtemplate{\includegraphics[width=\paperwidth]{GPGC}}%
\begin{frame}{Kitten}
\begin{itemize}
\item 1
\item 2
\item 3
\end{itemize}
\end{frame}
}

\begin{frame}{Summary}
\begin{itemize}
\item 1
\item 2
\item 3
\end{itemize}
\end{frame}

\end{document}