\DocumentMetadata{tagging = on}
\documentclass{ltx-talk}

\begin{document}

\begin{frame}
  Some text
\end{frame}

\AddToHook{shipout/background}
  {\put(0cm, -\paperheight)
      {\includegraphics[width = \paperwidth, height = \paperheight]
          {example-image-a}}}

\begin{frame}
  Some text
\end{frame}

\end{document}