Lindsay Stripped Down in Secret Fashion Shoot

8/4/2010 11:00 AM PDT

Lindsay Lohan -- Stripped Down in Muse Photo Shoot Video - Marc Ecko

We're sure there's a reason Lindsay Lohan threw on a ninja mask and flashed some underboob for a secret shoot for Marc Ecko, but it doesn't really matter ... we got the pics, so check 'em out already!

OK, now to the facts -- It was all part of Marc Ecko's secret "Muse" campaign directed by Markus Klinko and Indrani -- a secret he's had to keep under wraps for several months ... hence the mask.

TMZ has learned Lindsay posed for the shoot back in May -- right around the time she was getting into trouble with Judge Marsha Revel.

We're told Marc chose Lindsay because he wanted someone "whom fascinated us -- for better or worse."

Well, mission accomplished.