\newcommand\ct[2][\@empty]{% 

\makeatletter
\def\PicTure{\@ifnextchar[\@pic@ture\@@pic@@ture}
\def\@pic@ture[#1]#2{\includegraphics[width=#1cm]{#2}}
\def\@@pic@@ture#1{\includegraphics{#1}}
\makeatother