[DinkyDogg]

That's one way to do it, but not my favorite. I usually make a material with a lambertian shader (or just set specularity and glossiness to 0), add the reference image as the diffuse map, copy it to the self illumination channel, hit the button for "view material in viewport", then apply it to a plane with the same aspect ratio as the image. That way you can move it around more easily. You can freeze it so that you wont select it by accident, but first you should go to the display tab and make sure that "Display frozen in gray" is unchecked.

Your way is probably easier, but I like the added flexibility this method gives me. If you do it this way, it also shows up in the renders unless you hide it.


EDIT - Doh! I only read the beginning of your post. The method I just outlined is the planes method you suggested. Good for blueprints and stuff, because you can match up the planes in the different orthographic viewports.