Although there's no easy way to create a watermark using the basic iPhoto program, Apple does offer an iPhoto add-on called Impression that allows you to do so. This watermark can be a barcode, a logo, or text. Watermarks are useful if you wish to keep your digital images from being stolen on the Internet or used without your permission.
Go to the Apple Webpage and search the site for Impression. Download the program and install it.
Open iPhoto. Go to File, then Open. Find your image and open it in iPhoto.
Go to File and select Export. Select your photo from the list in the Export pop-up window.
Click on the Impression button in the Export pop-up window. Use the Impression buttons to select a watermark image and place it where you like on your image. The source file for your watermark can be PSD, PNG, or RTF files.
Export your watermarked image as a new file.