DMCA and EUCD are the Digital Millennium Copyright Act and the European Union Copyright Directive.
They are directives put into place to help copyright holders defend their rights on-line. The moment you make an original work such as taking a photo, writing a story or making a drawing, you automatically become the copyright holder of that work. There are other aspects to be considered, such as model rights for photographers and trademarks for characters you might want to draw or write about. But assuming you have a basic knowledge of copyright and actually own the rights to the work, you can file a DMCA/EUCD to stop people from reposting your (edited) work.

Some sites, such as DeviantArt or Tumblr, will have on-site possibilities and forms to file a DMCA/EUCD complaint.
In some cases you will have to contact a hosting company via E-mail, for which you will find a form letter below.

Be sure to edit accordingly.

_________________________________________________________________________________________________________________

To Whom It May Concern,

I am writing to you to avail myself of my rights under the Digital Millennium Copyright Act/European Union Copyright Directive¹. I wish to report an instance of what I feel in good faith is an instance or Copyright Infringement. The infringing material appears on the Service for which you are the designated agent.

1. The material which I contend belongs to me, and appears illegally on
the Service is the following: [description of your work, i.e. portrait photo of woman in red shirt]
2. The material appears at the website address:
[URL]
And can be found at these specific locations:
[specific URL to a post or direct link to the image]
Original:
[URL to where you’re hosting your work]
3. My contact information is as follows:
[Name, address etc.²]
4. I have a good faith belief that the use of the material that appears on the service is not authorized by the copyright owner, it’s agent, or by operation of law.
5. The information in this notice is accurate as the copyright owner. I declare under the perjury laws that this notification is true and correct.

– Signed, [your name]

_________________________________________________________________________________________________________________

¹ Depending on where their servers are, the USA or Europe, remove Digital Millennium Copyright Act or European Union Copyright Directive.
² Be careful about handing out your personal information to people who might have malicious intent, contact hosting companies and the like directly.