%
% Check if the image file (#1) exists. If so, it switches to #2, else to #3
% It assumes that the graphics package \Gin@extensions macro is defined.
%
\long\def\imgexists#1#2#3{%
  \begingroup%
  \let\img@file\relax%
  \filename@parse{#1}%
  \ifx\filename@ext\relax%
    \@for\Gin@temp:=\Gin@extensions\do{%
    \ifx\img@file\relax%
      \IfFileExists{#1\Gin@temp}{\def\img@file{#1}}{}%
    \fi}%
  \else
    \IfFileExists{#1}{\def\img@file{#1}}{}%
  \fi%
  \ifx\img@file\relax#3\else#2\fi%
  \endgroup%
}

%
% The image keeps its natural size if lower than the max boundaries
% Now it's fully based on internal graphics macros
%
% Macros defining the maximum lengths
\def\imgmaxwidth{\textwidth}
\def\imgmaxheight{\textheight}

% Real image sizes
\def\imgrwidth{\Gin@nat@width}
\def\imgrheight{\Gin@nat@height}
\def\imgevalsize#1{}%

% Use the maximum possible size
\def\imgwidth{%
  \ifdim\Gin@nat@width>\imgmaxwidth
    \imgmaxwidth
  \else
    \Gin@nat@width
  \fi
}

\def\imgheight{%
  \ifdim\Gin@nat@height>\imgmaxheight
    \imgmaxheight
  \else
    \Gin@nat@height
  \fi
}