Modifica tema Beamer

Buonasera a tutti,

eccomi qui con un altro dubbio.

Mi sono creato un tema Beamer personalizzato e sono abbastanza soddisfatto del risultato. Vorrei solo aggiungere un ultimo dettaglio, ovvero una filigrana come sfondo dalla seconda slide in poi.

Per adesso ho risolto inserendo dopo la prima slide:
`\usebackgroundtemplate{\includegraphics[width=\paperwidth]{cherubino.jpg}}`

però non ho capito come gestirlo anche nel file beamerouterthemePisa.sty. Vi metto il codice che ho del file citato.

`\mode

% Image definitions
% ==============================================================================
\pgfdeclareimage[width=\paperwidth]{titleBackground}{front}
\pgfdeclareimage[width=\paperwidth]{pageBackground}{foot}
\pgfdeclareimage[width=3.6cm]{logounipi}{unipi_logo}
\pgfdeclareimage[width=0.55cm]{logounipiwhite}{unipi_logo_white}
\pgfdeclareimage[width=\paperwidth]{ftitle}{title}
\pgfdeclareimage[width=\paperwidth]{sfondo}{cherubino}
% Actual beamer definitions
% ==============================================================================

\defbeamertemplate*{title page}{pd theme} {
\vspace{3ex}
\centering\usebeamerfont{title}\usebeamercolor[fg]{title}\inserttitle\par
\ifx\insertsubtitle\@empty
\else
\vskip0.4ex
{\centering\usebeamerfont{subtitle}\usebeamercolor[fg]{subtitle}\insertsubtitle\par}
\fi

\vspace{5ex}
\raggedright\usebeamerfont{author}\usebeamercolor[fg]{normal text}\insertauthor\par
\vspace{3ex}
\raggedright \usebeamerfont{author} \usebeamercolor[fg]{normal text}\insertshortinstitute\insertdate\par
\vfill
}

\defbeamertemplate*{background canvas}{pd theme} {
\ifnum\c@framenumber=1
\pgfuseimage{titleBackground}
\fi

}

\defbeamertemplate*{footline}{pd theme} {
\ifnum\c@framenumber=1
\ifbeamerlogos
\pgftext[right,bottom,at=\pgfpoint{0.99\paperwidth}{0.02cm}]{\pgfuseimage{logounipi}}
\fi
\else

\pgftext[right,bottom,at=\pgfpoint{\paperwidth}{0cm}]{\pgfuseimage{pageBackground}}
\ifbeamerlogos
\pgftext[right,bottom,at=\pgfpoint{0.999\paperwidth}{0.005cm}]{\pgfuseimage{logounipiwhite}}
\fi

\pgftext[right,bottom,at=\pgfpoint{0.94\paperwidth}{0.01\paperwidth}]{
\usebeamerfont{frame number in foot}
\usebeamercolor[fg]{frame number in foot}\insertframenumber{} of \inserttotalframenumber
}
\fi
}

\defbeamertemplate*{headline}{infolines theme}{
\ifnum\c@framenumber=1
\begin{beamercolorbox}[wd=\paperwidth,ht=2.25ex,dp=1ex,right]{caso}
\hspace*{2ex}
\end{beamercolorbox}%
\else
\leavevmode%
\hbox{%
\begin{beamercolorbox}[wd=.5\paperwidth,ht=2.25ex,dp=1ex,right]{section in head/foot}%
\usebeamerfont{section in head/foot}\insertsectionhead\hspace*{2ex}
\end{beamercolorbox}%
\begin{beamercolorbox}[wd=.5\paperwidth,ht=2.25ex,dp=1ex,left]{subsection in head/foot}%
\usebeamerfont{subsection in head/foot}\hspace*{2ex}\insertsubsectionhead
\end{beamercolorbox}}%
\vskip0pt%
\fi
}

\setbeamertemplate{frametitle}
{
\leavevmode
\pgftext[right,bottom,at=\pgfpoint{11.8cm}{-1.04ex}]{\pgfuseimage{ftitle}}
\hspace{-0.8cm}\usebeamerfont{frametitle}\insertframetitle\par
}

\mode
`

Se serve posso caricare tutta la cartella del tema.

EDIT:
Scusate era stupido, ma sono stanco tra il lavoro e la tesi, bastava un
`\defbeamertemplate*{background canvas}{pd theme} {
\ifnum\c@framenumber=1
\pgfuseimage{titleBackground}
\else
\pgfuseimage{sfondo}
\fi
`

Lascio aperto il post nel caso servisse a qualcuno?