How can I make a set of vertices part of an existing plane with an imported image? I imported an image as a plane in order to place vertices marking all the key contours on it. After a while I realized I didn't know how to make those vertices part of the plane they were on. 

If I can't move the vertices on the z axis, and have the image move with them, I can't use the image as a reference the way I'd planned and the whole thing becomes much more difficult.

Is there a way to embed those vertices in the plane that has the image? They are already sitting on it. Or maybe a way to import the image again and integrate the vertices with it then?
the only thing that comes to mind is subdividing the plane in edit mode. then moving your new vertices where you want them.

this may become a little more complicated if your image skews while doing this.

let me know if you need clarification. i can post some screen shots.