\mode<presentation>

\setbeamertemplate{background}{
\ifnum\thepage>1
\includegraphics[height=\paperheight]{beamerbackgroundtrex.png}
\else
\includegraphics[height=\paperheight]{beamertitletrex.png}
\fi
}

% Title page
\defbeamertemplate*{title page}{trex}[1][]
{ 
   \vskip2cm%
    \begin{beamercolorbox}[wd=6.cm,leftskip=-0.5cm,#1]{title page header}
      \usebeamerfont{title}\inserttitle\par%
    \end{beamercolorbox}%
    \vskip0.2cm%
    \begin{beamercolorbox}[wd=6.cm,leftskip=-0.5cm,#1]{author}
      \usebeamerfont{author}\insertauthor%
    \end{beamercolorbox}
     \vskip0.5cm%
    \begin{beamercolorbox}[wd=6cm,leftskip=-0.5cm,#1]{date}
      \usebeamerfont{author}\insertdate%
    \end{beamercolorbox}
     \vskip0.3cm%
    \begin{beamercolorbox}[wd=7cm,leftskip=-0.5cm,#1]{institute}
      \usebeamerfont{author}{\tiny \insertinstitute}%
    \end{beamercolorbox}
  \vfill
}

\mode
<all>