We do recommended that you mark your work with a copyright notice, as this makes it clear that copyright exists, and helps to deter infringement. For traditional prints, it is customary to use a stamp to mark the copyright notice and the copyright owners contact details on the back of the print. For electronic images, it is also possible to include the copyright/contact details in the file properties. Under Windows for example, right clicking on a image will allow you to bring up the properties dialogue where you may enter details about the file, (though this will only work with certain file types). More typically, your image software will provide a way to insert comments into the file; this is preferred as these are harder to remove.