You need to use the starred version *
of the figure
environment:
\documentclass[twocolumn]{article}
\usepackage[showframe]{geometry}% http://ctan.org/pkg/geometry
\usepackage{lipsum}% http://ctan.org/pkg/lipsum
\usepackage{graphicx}% http://ctan.org/pkg/graphicx
\begin{document}
\lipsum[1-2]
\begin{figure*}
\includegraphics[width=\textwidth,height=4cm]{tiger}
\caption{This is a tiger.}
\end{figure*}
\lipsum[3-10]
\end{document}
This will usually flush the figure
to the top of the following page, so there's not much control left to the user for movement. However, this may just be dependent on the user output.
In my opinion, you may be better off (for ease of use) with the multicol
package. Then you can specify text (and sectional commands) in a multicols
environment (with a mandatory argument specifying the number of columns), while specifying your figures in the usual way. The layout is different, since the content flows now with the multicols
environment. However, this may also be user preference.
Here's a minimal working example:
\documentclass{article}
\usepackage[showframe]{geometry}% http://ctan.org/pkg/geometry
\usepackage{lipsum}% http://ctan.org/pkg/lipsum
\usepackage{multicol}% http://ctan.org/pkg/multicols
\usepackage{graphicx}% http://ctan.org/pkg/graphicx
\begin{document}
\begin{multicols}{2}
\lipsum[1-2]
\end{multicols}
\begin{figure*}[h]
\includegraphics[width=\textwidth,height=4cm]{tiger}
\caption{This is a tiger.}
\end{figure*}
\begin{multicols}{2}
\lipsum[3-4]
\end{multicols}
\end{document}
\usepackage{graphicx}
you can \begin{figure*}[!ht] \centering \includegraphics{figures.png} \caption{Caption!} \label{fig:test} \end{figure*}