There is also a add-on in 2.5 called Import Images as planes.

Enable the Add on in the User Preferences and you can import an image as a seperate object who's aspect ratio will match the chosen image. This way you can move said (image) object for alignment and the image will remain in view whatever the camera position.

Once set up, just disable the selection and render options in the outliner and your ready to go. Very useful to avoid fiddling around with the background image options.