\hspace{-\marginparwidth}\hspace{-\marginparsep}\includegraphics{...}
If you don't want to insert this by hand after you know which page you
land on, use the \label for the caption, and test with ifthen.sty...
\leftline{%
\ifthenelse{\isodd{\pageref{fig:wideone}}%
{}{\hspace{-\marginparwidth}\hspace{-\marginparsep}}%
\uncludegraphics{...}
}