Adding copyright information to the metadata by itself is not going to protect your images as it is easy to strip out the metadata information. If you really wanted to protect your copyright you could draw some text or apply a watermark onto your image (not the original of course) which is a much stronger level of protection.
iMagine Photo can be used to apply watermarks or draw text onto the image, and it can also be used to set the metadata copyright information, all much faster than using photoshop.
To add a copyright message to the metadata see (assumes jpeg):
on AddCopyrightToFile(copyrightMessage, theFile)
tell application "iMagine Photo"
set thisImporter to import graphic theFile
tell thisImporter to make exporter with properties {export file type:"JPEG", export file location:theFile}
set the export exif user data of thisImporter to theFile
set the export exif data of thisImporter to {{exif type:copyright, exif unicode: copyrightMessage}}
export thisImporter
close thisImporter
end tell
end AddCopyrightToFile