From the Terms of Service

Copyright Infringement

If you believe a post infringes upon your copyright, please send an email to the webmaster with the following pieces of information:

• The URL of the infringing post.
• Proof that you own the copyright.
• An email address that will be provided to the person who uploaded the infringing post to facilitate communication.

I don't think a flag is the place to report this. Message the administrator if you own the copyright and want this work down.