I don't know if this is the best way or not but here goes. I usually map my source image to a plane. If I am modeling with nurbs I use a poly and if I am using polys I create a nurbs plane. That way if I don't want to see the plane I can simply toggle that type of geometry off and still see my model.
I open the image to get the dimensions. If the image is 512*1024 pixels I scale the plane to .5*1 so that the image maps to it without distortion. Now I group the plane so that I can scale the group with the selection handle if I need to and I don't have to worry about the proportions getting messed up. Just select the plane and hit the up arrow key to move the selection up to the group. Then I orient the planes into the view that matches them. front and front, side and side.
Or you can adjust where the center is in the orthographic views. If you want the easy way out. Just open the image plane attribute editor View->Image Plane->Image Plane Attributes and look under the "placement extras" section.
Hope this helps some.
ww