To print on a paper bag, invert the image (across horizon), click on "Custom" and enter the dimensions (5.2x10.2). Rename the custom print, "paper bag". Hit OK and you're all set. Next time, you'll just have to go to paper size and select "paper bag".
This is a great site for using Smart Boards in the classroom. It contains links to other sites, resources and interactive activities to engage learners using the Smart Board. For my favorite colleagues!