Fullpage picture in two column layout

It is not elegant, but with float package loaded you can use:

\begin{figure}[H]
\onecolumn\includegraphics{arc}
\end{figure}
\twocolumn

But you have to place this piece of code to exact locetion in source code. Otherwise you'll get pagebreak anywhere in twocolumned page, then page with image image.

Use the figure* environment. So instead of

\begin{figure}[ht] % I typically use ht
\centering
...
\end{figure}

you should use

\begin{figure*}[ht]
\centering
...
\end{figure*}

This also works for tables (i.e. table*). Hope this helps! Consider this link for more information

To supplement @Crowley's answer, to avoid pagebreak after implementation. Instead of using \twocolumn, use this package instead \usepackage{multicol}. Then,

  \begin{multicols}{2}
   \section Write or place anything you want
    \end{multicols}

This works for me!