The practice of patent, trademark and copyright law is a specialized field of law and usually only attorneys who practice in the area of infringement of those areas exclusively know what they are doing
however with regard to copyrights in general:
Do I Have To Copyright My Photos? :: Digital Photo Secrets
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How do I copyright my photos?
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Copyright in General
What is copyright?
Copyright is a form of protection grounded in the U.S. Constitution and granted by law for original works of authorship fixed in a tangible medium of expression. Copyright covers both published and unpublished works.
What does copyright protect?
Copyright, a form of intellectual property law, protects original works of authorship including literary, dramatic, musical, and artistic works, such as poetry, novels, movies, songs, computer software, and architecture. Copyright does not protect facts, ideas, systems, or methods of operation, although it may protect the way these things are expressed. See Circular 1, Copyright Basics, section "What Works Are Protected."
How is a copyright different from a patent or a trademark?
Copyright protects original works of authorship, while a patent protects inventions or discoveries. Ideas and discoveries are not protected by the copyright law, although the way in which they are expressed may be. A trademark protects words, phrases, symbols, or designs identifying the source of the goods or services of one party and distinguishing them from those of others.
When is my work protected?
Your work is under copyright protection the moment it is created and fixed in a tangible form that it is perceptible either directly or with the aid of a machine or device.
Do I have to register with your office to be protected?
No. In general, registration is voluntary. Copyright exists from the moment the work is created. You will have to register, however, if you wish to bring a lawsuit for infringement of a U.S. work. See Circular 1, Copyright Basics, section “Copyright Registration.” Copyright in General (FAQ) | U.S. Copyright Office
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the problem is what happens when you give up your right by posting photos where the sites indicate you give up your right to copyright by using the site
one way to have an argument against giving up your right may be by posting " all rights reserved " with the post ( check my signature block ) no guaranty that would work however
some sites, such as flickr.com you can actively reserve the right of copyright when you post the photo
BTW, I am a licensed attorney but I don't practice in the field of patent, trademark or copyright infringement