Adding an image to a pre-made beamer in LaTeX

I would like to use a beamer template named Argüelles (https://github.com/piazzai/arguelles/?tab=readme-ov-file) and add a picture to the title page. The definition of the title page is :

% title page
\defbeamertemplate*{title page}{Arguelles}{
  \vfill\begin{beamercolorbox}{inverted text}
    {\usebeamerfont{title}\inserttitle}\par
    {\usebeamerfont{subtitle}\insertsubtitle}\par\bigskip
    {\usebeamerfont{event}\insertevent}\par
    {\usebeamerfont{date}\insertdate}\par\bigskip
    {\usebeamerfont{author}\insertauthor}\par\smallskip
    {\usebeamerfont{institute}\insertinstitute}\par
    {\usebeamerfont{email}\insertemail}\par
    {\usebeamerfont{homepage}\inserthomepage}\par
    {\usebeamerfont{github}\insertgithub}
  \end{beamercolorbox}
  \addtocounter{framenumber}{-1}
}

and we use it :

\begin{document}
\frame[plain]{\titlepage}

so I thought adding the tikz package and adding the picture directly in the template file would work however I don't why it doesn't :

  \end{beamercolorbox}
  \begin{tikzpicture}[remember picture,overlay]
  \node[anchor=south east, xshift=-0.5cm, yshift=0.5cm] at (current page.south east) {
    \includegraphics[width=2cm]{obs.png}
  };
  \addtocounter{framenumber}{-1}

Else, I don't know if it is possible to do it directly on my .tex file...

Thank you !

Here is a compilable example :

\documentclass[compress,12pt]{beamer}

\usetheme{Arguelles}

\title{Argüelles}
\subtitle{Simple, typographic beamer theme}
\event{}
\date{}
\author{Place Holder}
\institute{University of \TeX}
\email{name@domain.com}
\homepage{www.mywebsite.com}
\github{username}

\begin{document}

\frame[plain]{\titlepage}

\Section{Demo}

\begin{frame}
      \frametitle{A frame with title and subtitle}
      \framesubtitle{Subtitle here}
      Lorem ipsum dolor sit amet, consectetur adipiscing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua \par
      Itemized list:
      \begin{itemize}
            \item Lorem ipsum
            \item Dolor sit amet
                  \begin{itemize}
                        \item Consectetur
                        \item Adipiscing elit
                  \end{itemize}
            \item Sed do eiusmod
                  \begin{itemize}
                        \item Tempor incididunt
                              \begin{itemize}
                                    \item Ut labore et dolore
                                    \item Magna aliqua
                              \end{itemize}
                  \end{itemize}
      \end{itemize}
\end{frame}
\end{document}

You can use \addtobeamertemplate{title page}{<code before>}{<code after>} to add additional code to the title page. Make sure:

• that you have a \end{tikzpicture}
• compile two times to create page coordinates like current page.south east

\documentclass[compress,12pt]{beamer}

\usetheme{Arguelles}

\title{Argüelles}
\subtitle{Simple, typographic beamer theme}
\event{}
\date{}
\author{Place Holder}
\institute{University of \TeX}
\email{name@domain.com}
\homepage{www.mywebsite.com}
\github{username}

\addtobeamertemplate{title page}{}{%
  \begin{tikzpicture}[remember picture,overlay]
    \node[anchor=south east, xshift=-0.5cm, yshift=0.5cm] at (current page.south east) {
      \includegraphics[width=2cm]{example-image-duck}
    };
  \end{tikzpicture}%
}

\begin{document}

\begin{frame}[plain]
\titlepage
\end{frame}

\end{document}