No output when inserting a background image

Question

I would like to insert a background image contained in my_background_file.png in the title page of a presentation. Why doesn't the following code properly work? I simply get a title page, without any image on the background.

\documentclass{beamer}
\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc}
\title{YYYY}
\author{D.S.}
\institute{Some University}
\usepackage{transparent}
\usepackage{eso-pic}
\newcommand\BackgroundPic{%
\put(0,0){%
\parbox[b][\paperheight]{\paperwidth}{%
\vfill
\centering
{\transparent{0.4} \includegraphics[width=\paperwidth,height=\paperheight,%
keepaspectratio]{my_background_file.png}}%
\vfill
}}}
\begin{document}
\AddToShipoutPicture*{\BackgroundPic}
    \titlepage
\end{document}

What's wrong with that?

Answer 1

Try this.

\documentclass{beamer}
\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc}
\title{YYYY}
\author{D.S.}
\institute{Some University}
\usepackage{transparent}
\usebackgroundtemplate{%
\transparent{0.4}\includegraphics[width=\paperwidth,height=\paperheight]{example-image-A}
}
\begin{document}
    \titlepage
\end{document}

To change the background of each frame you could try this code:

\documentclass{beamer}
\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc}
\title{YYYY}
\author{D.S.}
\institute{Some University}
\usepackage{transparent}
\begin{document}
{%
\usebackgroundtemplate{%
\transparent{0.4}\includegraphics[width=\paperwidth,height=\paperheight]{example-image-A}}
\begin{frame}
    \titlepage
\end{frame}
}
{%
\usebackgroundtemplate{%
\transparent{0.4}\includegraphics[width=\paperwidth,height=\paperheight]{example-image-B}}
\begin{frame}
    One
\end{frame}
}
{%
\usebackgroundtemplate{%
\transparent{0.4}\includegraphics[width=\paperwidth,height=\paperheight]{example-image-C}}
\begin{frame}
    Two
\end{frame}
}
\end{document}