Full width image in beamer?

Without having access to your beamer theme nor to the image, it is difficult to test. But if you know the size of your margins the following (dirty but simple) solution might work.

\vspace*{-6.5mm}    
\begin{frame}[plain]
    \hspace*{-19mm}
    \includegraphics[width=\paperwidth]{picture}
\end{frame} 

(to be used without any geometry resetting commands)

---

Or without manually fiddling around with the size of the margins:

\documentclass{beamer}

\makeatletter
\newlength\beamerleftmargin
\setlength\beamerleftmargin{\Gm@lmargin}
\makeatother

\begin{document}

\begin{frame}[plain]
    \hspace*{-\beamerleftmargin}%
    \includegraphics[width=\paperwidth,width=\paperwidth]{example-image}
\end{frame} 

\end{document}

A plain frame with a background image can be locally applied without need for changing and restoring geometry changes

\documentclass{beamer}   
\mode<presentation>
 {
\usetheme{Berkeley} %<-- We don't have `myslides` theme
 }

\begin{document}

\begin{frame}[fragile]
\frametitle{}

\begin{center}
\huge Introduction?
\end{center}
\end{frame}

{%<--- Start local changes
\setbeamertemplate{navigation symbols}{}
\usebackgroundtemplate{\includegraphics[width=\paperwidth]{example-image}}
\begin{frame}[plain]
\end{frame}
}%<---- Finish local changes

\begin{frame}[fragile]
\frametitle{}
\begin{center}
\huge Next slide?
\end{center}
\end{frame}
\end{document}

A very simple solution using tikz:

\documentclass{beamer}

\usepackage{tikz}

\begin{document}

\begin{frame}[plain]
    \begin{tikzpicture}[remember picture,overlay]
        \node[at=(current page.center)] {
            \includegraphics[width=\paperwidth, height=\paperheight]{example-image}
        };
    \end{tikzpicture}
\end{frame}

\end{document}

[has to be compiled two times to get the coordinates right]