%% https://tex.stackexchange.com/questions/26458/beamer-background-image-centered
%% https://tex.stackexchange.com/questions/74038/transparent-image-background-in-beamer
\usepackage{tikz}
\usebackgroundtemplate {
  \vbox to \paperheight {
    \vfil
    \hbox to \paperwidth {
      \hfil
      \tikz\node[opacity=0.25] {
        \includegraphics[width=0.75\textwidth]{imgs/confine,background.pdf}
      };
      \hfil
    }
    \vfil
  }
}
