To composite an image on top of the current image, set that image as the secondary, select RubThrough mode using the Paint Mode Selector 
, and rub through to the image using the Airbrush, or filling the paint layer 
. Use the Position tool 
to place the image. See the instructions in the Tools section for different ways of using this tool. To add a new image on top, create a new paint layer using the Layer->New->Paint Layer menu item, and repeat the above steps (although you shouldn't need to change the mode to RubThrough as it will remember from before).