\mode<presentation>

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

% Title page
\defbeamertemplate*{title page}{pterosor}[1][]
{
   \vskip3cm%
    \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}
     \vskip0.3cm%
    \begin{beamercolorbox}[wd=6.cm,leftskip=-0.5cm,#1]{date}
      \usebeamerfont{author}\insertdate%
    \end{beamercolorbox}
     \vskip0.3cm%
    \begin{beamercolorbox}[wd=7.cm,leftskip=-0.5cm,#1]{institute}
      \usebeamerfont{author}{\tiny \insertinstitute}%
    \end{beamercolorbox}
  \vfill
}

% Section page
\defbeamertemplate*{section page}{pterosor}{
  \centering
  \begin{minipage}{22em}
    \raggedright
    \usebeamercolor[fg]{section title}
    \usebeamerfont{section title}
    \insertsectionhead\\[-1ex]
    \par
    \ifx\insertsubsectionhead\@empty\else%
      \usebeamercolor[fg]{subsection title}%
      \usebeamerfont{subsection title}%
      \insertsubsectionhead
    \fi
  \end{minipage}
  \par
  \vspace{\baselineskip}
}


\mode<all>