Figure with an image

\documentclass{article}
\usepackage{graphicx}
\begin{document}
\section*{A figure with an image}

Figure~\ref{fig:plot} shows $\sin x \cos y$ on $[0, 2\pi]^2$ as colour bands. The image,
\texttt{plot.png}, is a file in this folder: the preview reads it from there.

\begin{figure}[ht]
  \centering
  \includegraphics[width=0.4\textwidth]{plot.png}
  \caption{The function $\sin x \cos y$: blue below zero, red above.}
  \label{fig:plot}
\end{figure}

As Figure~\ref{fig:plot} shows, the zero set is a grid of lines.
\end{document}