Ya’ll hashed this out pretty well, but thought I’d add that any metadata or file properties like EXIF or IPTC info can be deleted from the file. However, it is possible to add digital watermarks which imbed themselves into the data of the image. The image can then be tracked on the internet to some degree. The software and services to do so, however, seem cost prohibitive for most casual photography.
Carolyn Wright is a copyright attorney who has some resources on her website.
http://www.photoattorney.com/