add background image example

tutorial/frame_background.tex

+\setbeamertemplate{background}{\includegraphics[width=\paperwidth]{placeholder}}
+\begin{frame}
+    \frametitle{An image in the background}
+    \centering
+    Some text in front of the background image.
+\end{frame}
+\setbeamertemplate{background}{}

tutorial/part_examples.tex

@@ -67,6 +67,22 @@
         \end{center}
 \end{frame}
 
+\section{Backgrounds}
+\begin{frame}[fragile]
+    \frametitle{Background Images}
+    \framesubtitle{On Standard Frames}
+    \begin{itemize}
+      \item The next slide shows an image, embedded into the background of the frame layout.
+      \item The background image is automatically cropped to the frame dimensions.
+    \end{itemize}
+    \begin{block}{How to install a background image}
+        \scriptsize
+        \verbatiminput{frame_background}
+    \end{block}
+\end{frame}
+
+\input{frame_background}
+
 \section{Beamer Block Constructs}
 \subsection{Theorem, Proof}
 \begin{frame}