The problem is much simpler than you think it is. If you create a polygon plane and make a material for it. Plug the referrence image into this material. Create a new polygon plane for every referrence image you want in the scene. Temporary ones of course. When the modeling is done you can delete them, but I rarely do. Usually keep them in the file on a separate layer. It does not impact the filesize much.
Here's how.
1) Find out the width and height of the referrence image. Let's say it's 640 pixels by 480 pixels.
2) Create a polygon plane and in the width input 6.40 and height 4.80 to maintain scale of the source image.
3) When that is done you can scale the polyplane uniformly to any size you want and the image ratio will be kept.
4) You can position the planes and rotate them as needed. If you have a front and side view for instance.
5) Oh and don't forget to hit 6 on the keyboard to display textures on the polygon plane.
Good luck and have fun now.
PS: Some of the free video tutorials here might show you this technique in action.