If you want to post a warning, something similar to the following would be standard:
"© 2015 [Name of Copyright Owner] This work is protected by copyright and you may not copy, distribute, or modify it. Failure to comply exposes you to legal action for copyright infringement."If you want to give your warning a more dramatic vibe, you're also free to download and post the FBI's badge-like warning.
However, don't expect these warning to be much of a deterrent. As the RIAA and MPAA are already aware, copyright warnings have negligible effect on content piracy (just as alcohol and tobacco warnings have little effect on drinking and smoking).
Be Prepared. We think it's more important to be prepared for copyright infringement than to warn against it. For example, registering your work can provide benefits if you must chase an infringer (and the certificate of registration adds some gravitas to your cease and desist letters). You should also learn how to police for infringements -- for example, for free you can create a Google alert or a Google reverse image search. And of course, include your copyright notice (© [Year] [Name]) whenever possible.