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.
Visit Apple store and look for Impression app. Download the program and install it.
We never thought that Apple Mac computers will not infected by any malwares but According to this report http://news.drweb.com/show/?i=2341&lng=en&c=14 more than half a million Apple computers have installed the malware.
Doctor Web recommends all Mac users to download and install a security update released by Apple fromsupport.apple.com/kb/HT5228 to prevent infection of their systems by BackDoor.Flashback.39.