clipping_image.tex

% Answer for http://tex.stackexchange.com/a/193592/828 using http://commons.wikimedia.org/wiki/Frog#mediaviewer/File:Masked_tree_frog_head.JPG from Wikimedia

\documentclass{article}

\usepackage{graphicx}
\usepackage{tikz}
\usepackage{lipsum}
\usepackage[active,tightpage]{preview}

\PreviewEnvironment{tikzpicture}

\newcommand{\imsize}{\linewidth}
\newlength\imagewidth
\newlength\imagescale

\begin{document}

\lipsum[1]

\renewcommand{\imsize}{0.618\linewidth}
\pgfmathsetlength{\imagewidth}{\textwidth}%
\pgfmathsetlength{\imagescale}{\imagewidth/851}%
\begin{figure}[htb]
\begin{tikzpicture}[x=\imagescale,y=-\imagescale]
	\clip (851/2, 567/2) circle (567/2);
	\node[anchor=north west, inner sep=0pt, outer sep=0pt] at (0,0) {\includegraphics[width=\imagewidth]{Masked_tree_frog_head}};
\end{tikzpicture}
\caption{My image}
\end{figure}

\lipsum[1]

\end{document}