How to insert a background image in a beamer frame?

I used images Sir Isaac Newton (rename it as newton.jpg) and Kitten (rename it as kitten.jpg)

The code snippet below is self-explanatory and I compiled using pdflatex.exe because of .jpg format.

\documentclass[bigger]{beamer}
\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc}

\usepackage{graphicx}


%Global Background must be put in preamble
\usebackgroundtemplate%
{%
    \includegraphics[width=\paperwidth,height=\paperheight]{newton.jpg}%
}




\begin{document}

\begin{frame}{Introduction}
\begin{itemize}
\item 1
\item 2
\item 3
\end{itemize}
\end{frame}


% Local background must be enclosed by curly braces for grouping.
{
\usebackgroundtemplate{\includegraphics[width=\paperwidth]{kitten.jpg}}%
\begin{frame}{Kitten}
\begin{itemize}
\item 1
\item 2
\item 3
\end{itemize}
\end{frame}
}

\begin{frame}{Summary}
\begin{itemize}
\item 1
\item 2
\item 3
\end{itemize}
\end{frame}

\end{document}

Write \usebackgroundtemplate before the frame, not within:

\end{frame}
% Now we install the new template for the following frames:
{\usebackgroundtemplate{%
  \includegraphics[width=\paperwidth,height=\paperheight]{crayons}} 
\begin{frame}
...
\end{frame}}
% Now we install another template, effective from now on:
\usebackgroundtemplate{...}

Ensure that \includegraphics is able to find the image file, since you used ../images/crayons.png.

Edit: The \usebackgroundtemplate should be enclosed with curly braces along with frame (or frames) which it should affect