I'm editing a text provided by my client as a LyX file that includes graphics and other files that I don't have. I've "commented out" the graphics by putting in a custom preamble that substitutes the file-loading definitions with macros that just quote their arguments. However, the \includegraphic macro throws up an error in the two tabular environments where it is used, complaining that \endfirsthead was never defined. What is this macro, and why is it interfering with mine?

The preamble:

\usepackage{natbib}
\def\fileloc{/some/path/stylefile}
\newif\iftextproof
\IfFileExists\alifefileloc\textproofalse\textprooftrue
\iftextproof
  \usepackage{./stylefile}
 %natbib subst:
  \renewcommand\cite[1]{{\tt #1}}
 %suppress graphics includes:
  \def\quotearg#1{\expandafter\string\csname #1\endcsname}
  \newcommand\gobbleIG[2][*void*]{{\footnotesize \quotearg{#2}}}
  \newcommand\gobbleSF[2][*unnamed*]{{\bf #1}}
  \let\includegraphics=\gobbleIG
  \let\subfigure=\gobbleSF
\else
  \usepackage\fileloc
\fi