\documentclass[a4paper,twocolumn]{article}
\usepackage{lipsum,graphicx}
\newcommand{\mydimright}{%
\dimexpr\paperwidth-1in-\hoffset-\oddsidemargin-
\columnwidth-\columnsep\relax}
\newcommand{\mydimleft}{%
\dimexpr1in+\hoffset+\oddsidemargin+\columnwidth\relax}
\begin{document}
\lipsum[1]
\begin{flushleft}
\makebox[\columnwidth][r]{%
\includegraphics[height=1cm,width=\mydimleft]{try}}
\end{flushleft}
\lipsum[1-5]
\begin{flushleft}
\makebox[\columnwidth][l]{%
\includegraphics[height=1cm,width=\mydimright]{try}}
\end{flushleft}
\end{document}
There is no immediate access to the right margin width, which has to be computed in an indirect way.
With the command \layout of the layout package, one can look at the relevant lengths. I've put also \hoffset, because some classes might change it.
Here try is the graphic file I always use for tests. I've used flushleft just in order to avoid LaTeX trying to be smart with figure placement.
-- Extended version for automatic placement in the figure environment --
Here it is, following Martin's suggestion; the command \touchmarginincludegraphics works like its sibling \includegraphics except, of course, that the keyword width should not be specified.
\documentclass[a4paper,twocolumn]{article}
\usepackage{lipsum,graphicx}
\makeatletter
\def\tmig@r{\dimexpr\paperwidth-1in-\hoffset-
\oddsidemargin-\columnwidth-\columnsep\relax}
\def\tmig@l{\dimexpr1in+\hoffset+\oddsidemargin+\columnwidth\relax}
\newcommand{\touchmarginincludegraphics}[2][]{%
\begingroup
\if@firstcolumn
\def\tmig@place{r}%
\else
\def\tmig@place{l}%
\fi
\makebox[\columnwidth][\tmig@place]{%
\includegraphics[#1,width=\@nameuse{tmig@\tmig@place}]{#2}}
\endgroup}
\makeatother
\begin{document}
\lipsum[1]
\begin{figure}[!h]
\touchmarginincludegraphics[height=1cm]{try}
\end{figure}
\lipsum[1-4]
\begin{figure}[!h]
\touchmarginincludegraphics[height=1cm]{try}
\end{figure}
\lipsum[2]
\end{document}
\makebox[\columnwidth][r]{\includegraphics[width=\dimexpr\columnwidth+\rightmargin\relax]{image}}in this case. I'm not sure about the margin length, though. – Martin Scharrer May 13 '11 at 15:50\rightmarginis definitely wrong; it's a parameter for lists. – egreg May 13 '11 at 16:19