The Blender freestyle rendering engine does not seem to be able to take into account the ‘edges’ created by a transparent texture.
Set up a plane and texture it with a picture so that the texture has an arbitrary outline surrounded by fully transparent areas (as described in this tutorial: A plane textured with an image with transparency tutorial http://overshoot.tv/wiki/blender/plane_textured_image_transparency_tutorial): freestyle will render the shape of the plane, not that of the shape of the image texture.
Is this a known limitation?
Will freestyle one day take transparent texture into account?
http://overshoot.tv/ticket/3143