The Digital Millennium Copyright Act (DMCA) provides “safe harbor” for service providers that allow users to post content online. When a third party uses a service provider to post material online that may infringe copyright, the service provider is protected from liability for the infringement. In exchange for this protection, the service provider must agree to quickly remove allegedly infringing material when proper notice is given. Thus, if your photograph is posted online in a way that infringes on your copyright, in most cases you can issue a DMCA takedown notice to have it removed.
Note that the DMCA takedown process should not be abused. In fact, a federal appeals court held that copyright owners are required to consider whether a possible infringing use is actually a fair use before issuing a DMCA takedown notice.
In order to take advantage of the DMCA safe harbor provisions, an online service provider must designate an agent to receive notices of copyright infringement. There is a directory of agents on the Copyright Office website.
A DMCA takedown notice must be submitted to the service provider’s designated agent, and include the following information:
(i) A physical or electronic signature of a person authorized to act on behalf of the owner of an exclusive right that is allegedly infringed.
(ii) Identification of the copyrighted work claimed to have been infringed, or, if multiple copyrighted works at a single online site are covered by a single notification, a representative list of such works at that site.
(iii) Identification of the material that is claimed to be infringing or to be the subject of infringing activity and that is to be removed or access to which is to be disabled, and information reasonably sufficient to permit the service provider to locate the material.
(iv) Information reasonably sufficient to permit the service provider to contact the complaining party, such as an address, telephone number, and, if available, an electronic mail address at which the complaining party may be contacted.
(v) A statement that the complaining party has a good faith belief that use of the material in the manner complained of is not authorized by the copyright owner, its agent, or the law.
(vi) A statement that the information in the notification is accurate, and under penalty of perjury, that the complaining party is authorized to act on behalf of the owner of an exclusive right that is allegedly infringed. 
The definition of “online service provider” for DMCA purposes is very broad, and includes image-sharing websites like Instagram and Flickr, as well as website hosting companies. Major social media companies will provide information about how to issue a DMCA takedown notice, and may even provide an online form for that purpose. A list of links for several popular services is below. For an individual website or blog, you may have to find out who is hosting the website so that you know where to send the notice. One easy method is to visit whoishostingthis.com and type in the website address. When you have the host name, go to Google and type, “[host name] DMCA” to find the DMCA agent/instructions for that webhost.
DMCA Information for Popular Social Media and Webhosts
- Automattic (WordPress.com)
- Yahoo (Flickr)