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\documentclass{article}
\usepackage{graphicx}
\begin{document}
\includegraphics[width=0.5\textwidth]{file}
\end{document}
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\documentclass{article}
\usepackage{graphicx}
\begin{document}
\begin{figure}
\makebox[\textwidth][c]{\includegraphics[width=1.2\textwidth]{image}}%
\caption{Caption}
\label{fig:key}
\end{figure}
\end{document}