Roots Television has a must-watch video for genealogists trying to manage their digital photos. The video shows how to label your digital genealogy images so you and others can easily identify them later. There are plenty of free good programs available to change the canvas size and add text. This example uses GIMP. "GIMP" stands for GNU Image Manipulation Program and is free for Windows, Macintosh, and Linux.
The video is available at: http://www.rootstelevision.com/players/player_howto3.php?bctid=13970104001&bclid=958499738.