Every paper size has a different shape, and none probably match the shape of the images produced by your digital camera. This little utility will create a layer showing which parts of an image will fit on the paper when printed. A wide selection of print sizes are included. Using it will help you layout your image so that important pieces aren't cut off by the printer/lab.
This script-fu was developed under Gimp-2.0 and Gimp-2.4