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\newlength{\testwd}
\newcommand{\fitpic}[1]{%
\settowidth{\testwd}{\includegraphics{#1}}%
\message{#1 width=\the\testwd, page=\the\textwidth}%
\ifthenelse{\lengthtest{\testwd>\textwidth}}{%
\noindent\includegraphics[width=\textwidth]{#1}}{%
\centering\includegraphics{#1}\par}%
}