Possible copyright infringment - need help

[jetrim]

I don't want to go pointing fingers too quickly but I'm pretty sure a local studio snatched one of my images and is using it for advertising. Wondering how to proceed as there is the REMOTEST possibility that there may have been another photographer standing beside me at the time, taking a nearly identical shot (it was six months ago and there were 9 photographers plus a slew of assistants there). This is a big time commercial rental studio that does a lot of high end fashion work, so there is also the possibility that they photoshopped the top and right side to make it look as if it was not exactly the same. No licensing agreement was ever signed by me, and it was not buried in any legalese at the time of sign-up (I read that stuff carefully). Do I approach them and sk them to produce their original? Speak with a copyright attorney? Other options?

here's the evidence:
1 - My original (resized)
2 - Their Ad
3 - My image overlayed on their image in "difference" blend mode