[Tex/LaTex] Add content behind beamer background (or theme)

Question

I have a custom beamer template that frames the content of each slide, like the standard themes but with wiggly header/footer lines.

I add the background image of the template with

\setbeamertemplate{background}
 {\includegraphics[width=\paperwidth,height=\paperheight]{master.png}}

I now want the content to be behind the background image and the rest of the template (title, page number…). The template will then have a cut-out with the content visible underneath. The background image can have .png or .pdf transparency.

Kind of like:
http://en.wikipedia.org/wiki/Passe-Partout_%28framing%29

Does anyone have an idea how to move the content behind the background image?

Thank you for your help!

Answer 1

In order to have the title in front of the picture: How about leaving the traditional frametitle layer empty and including the frametitle in the footline layer?

\documentclass[t]{beamer}

\setbeamercolor{background canvas}{bg=blue!20}
\setbeamerfont{frametitle}{size=\fontsize{24}{28},series=\bfseries}
\setbeamercolor{frametitle}{fg=white}

\pgfdeclareimage[width=\paperwidth]{mybackground}{back.pdf}

\providecommand\insertframetitle{} 
\setbeamertemplate{footline}{%
    \begin{picture}(0,0)
        \put(0,0){%
            \pgfuseimage{mybackground}
        }
        \put(25,220){%
            \begin{minipage}[b][0mm][t]{100mm}
                \usebeamerfont{frametitle}{
                    \usebeamercolor[fg]{frametitle}{
                        \insertframetitle
                    }
                }
            \end{minipage}
        }
    \end{picture}
}

\setbeamertemplate{frametitle}{\vspace*{2.5cm}}

\begin{document}

    \begin{frame}
        \frametitle{Test Test Test}
        test test test test test test test test test test test test test test test test test test test test test test test test test test test test test test test test test test test test test test test test test test test test test test test test test test test test test test test test test test test test test test test test test test test test test test test test test test test test test test test test test test test test test test test test test test test test test  test
    \end{frame}

\end{document}