To be honest I don't see a copy of your image. I see use of the same photo that you found and that someone else has done thier own changes and colorations of.

It's very possible that the contributor used the same famous photo and changed it like you did. A stencil face of Obama from the same photo will render like the image you did.

It's not easy to make claims against a famous face and a computer program that can render a stencil like image and call it theft.

They may have been inspired by your work and tried their own hand at it without directly using your art.

I don't know how many times I've seen this picture on zazzle in the last few weeks (months?).
What makes you think that that person took your design?
*shakes head and goes off to take some more pictures of her own....*

The key word in copyright protection though is "original" one can't really claim to own the exclusive copyright on elements of a design which use imagery either in the public domain or from clip art sources that are available for any one to use whether freely or for a small fee.

So in this case the only aspect you could claim copyright on is in the precise way the image has been used or adapted. But even there adjustments such as high contrast and some commonly available digital filters which are widely used, will usually tend to give very similar standard results particularly if the input image is virtually the same.

So whilst I certainly wouldn't condone ripping someone else's design off, and generally think it's pointless anyway if a market is already well catered for. In this case though given the widespread and public nature of images of political candidates and the fairly standard high contrast treatment the images have been given, which is a common photo silk-screen printing style and was also popular in Pop Art. I think the evidence that the other contributor has actually ripped-off your design is purely circumstantial. The circumstance being that both designs contain substantial elements that are not original to start with, but rather acquired from public sources.

Problem is of course the law generally works on evidence. If there is no evidence of wrong doing, or all the evidence is purely circumstantial, accusations are simply based on supposition, or suspicion as opposed to hard evidence, there is simply no case. Of course your suspicions might be correct, but since there's actually no way of proving that apart from a signed confession from the other contributor it's probably pointless pursuing it, apart from perhaps from a publicity angle.