The Professional Photographers of America (PPA) have posted a very good article on their blog about what do photographers need to do in order to protect their copyright.
10 Ways for Photographers to Protect Their Copyright
The first thing they advise is marking your images as copyrighted. Another thing they advise is letting people know how to contact you so if someone comes across one of your photos somewhere in the web and they wish to license that photo, they know whom to contact and how. You can simply put your web site URL or even better a QR code which people can scan with their phone and go straight to your site or give you a call.
This is a must read article for every photographer.
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