Look for the image that you'd like to duplicate in your Layers Palette (it's at the bottom right side of the main window). Right click on it and choose Duplicate Layer. Click OK on the pop-out window. Do this as many times as the number of copies you'd like to make.
By the way, it may look that nothing happened when you do this because the images are stacked on top of each other. Try moving an image on top to see the image below it.