In Curvy 3D 4.0 you can use "Texture/Resize Cube to Image" to scale the cube to match your image (In 3.0 use the Height and Width sliders). The front and side will scale, you can squash the cube in the other axis to make it easier to work with.
Then set the Object to use "Fog" blending in Object Properties and use the Alpha Slider to fade the reference image out so you can see your model through it.
A lot of references are used in the Front and Side views - which you don't really want cluttering up the Perspective view. There is a quick Materials hack you can use to hide the reference planes from the perspective view... just add the PlanarFilter.png image as your "Base Light". This map is a transparent square with a small opaque dot in the middle - this lightmap will make the object transparent from every angle except direct straight-on viewpoints - just what we need for it to show in the exact front/side views, and be transparent from angled views in the Perspective view.
http://www.curvy3d.com/images/c4/PlanarFilter.png
