Adding Reference Images to Blender

See for source!

You can create an Empty->Image in the scene, and then in properties for the Empty, add your image to it. Tick the transparency to about 0.5, and enable X-ray in the Object Properties tab, and it should be always visible!

I usually like to put one for front view, and one for side view, showing them only if the camera is facing directly forward or on the side.