DMCA refers to the Diital Millenium Copyright Act (1998) of the United States.  One of the provisions of benefit to copyright owners, including photographers, is the Notice and Takedown procedures which aims to protect online platforms from copyright infringement liability of their users acting illegally and committing infringements.  Photographers are able to protect their works from illegal postings by issuing takedown notices under the DMCA to the relevant websites where the images are posted, the internet service provider (ISP), and the hosting company.  Apart from copyright owners of unauthorised photographic content, anyone portrayed in a photo can issue a takedown notice and this can impact portfolio sites if the photographer has signed a previous agreement limiting the use of their work. 

The way a DMCA Takedown Notice works is that whenever a copyright owner discovers that a website platform is hosting unauthorised copies of their work, the copyright owner can send a takedown notice, in a prescribed format to the designated copyright agent for the website platform.  This information is usually published in the Privacy Policy of the hosting website;  and who, once notified and is made aware of any infringement, is then obligated to remove the illegal copies “expeditiously” (usually within 24-48 hours), thereby absolving themelves of liability for monetary damages caused by the infrngement.  This only applies to websites hosted by US providers.

Information on the DMCA Agent for the website is not the easiest to locate and In practice a domain search may be necessary to establish the country where the website is being hosted and who the hosting provider is.  The EU has similar legislation and in my experience the platforms and websites in the US and EU respond expeditiously.  However, where the website and the hosting company are outside of these two jurisdictions it can be challenging to get a takedown notice observed.