A small workaround, but not perfect for sure (depending on the graphics extension, it might fail) -- I use the \IfFileExists in a slightly redefined \includegraphics command. It test for four extensions (all in lower case) (.eps,.pdf, .jpg and .png)
\documentclass{scrbook}
\usepackage{blindtext}
\usepackage{graphicx}
\usepackage{tcolorbox}
\let\StandardIncludeGraphics\includegraphics%
\renewcommand{\includegraphics}[2][]{%
\IfFileExists{#2.eps}{%
\StandardIncludeGraphics[#1]{#2}%
}{%
\IfFileExists{#2.pdf}{%
\StandardIncludeGraphics[#1]{#2}%
}{ % No, no .pdf, try *.jpg
\IfFileExists{#2.jpg}{%
\StandardIncludeGraphics[#1]{#2}%
}{
\IfFileExists{#2.png}{%
\StandardIncludeGraphics[#1]{#2}%
}{%
\begin{tcolorbox}[width=6cm,height=4cm,arc=0mm,auto outer arc]
\end{tcolorbox}
}
}
}
}%
%
}% End of command
\begin{document}
\chapter{First one}
\blindtext
\begin{figure}
\begin{center}
\includegraphics[width=10cm,height=8cm]{some_file}
\caption{My demo figure}
\end{center}
\end{figure}
\blindtext[2]
\end{document}
The mentioned file some_file does not exist at all in my paths.

\begin{figure} ... \end{figure}you can put something like\rule{4cm}{8cm}. This could also be 'automated' using conditionals... – jon May 09 '14 at 04:25