You can also apply your image to a flat, one-sided plane and set your dimensions on the y- and x- scale.  Afterwards, change the overall scale to whatever size you desire.  To avoid casting shadows on the plane, connect your image node in the material room to the ambient_color and set it's value to 1.  You can achieve the same affect by also connecting to alt_diffuse.  Make certain the plane's cast shadow is turned off on it's properties panel.  An example can be found here:

http://www.renderosity.com/mod/gallery/index.php?image_id=1884989&user_id=343328&np&np