If you spin the reels and create a unique screenshot (like I do), then it is undeniably yours. If you just use the screenshot sent to you by the affiliate programs, or if you take a screenshot of the initial game screen (before you spin), you should expect many other people to have the same screenshot on their sites.
I also recommend watermarking your images. Last week I watermarked all of my screenshots. Now, if Google picks up the images, or if someone (very stupidly) steals an image and uses it on their site, I’ll get free advertising thanks to the watermark. 
Screenshot and the part of the text of
xhttp://www.onlineslotmachines.biz/microgaming-slots/mega-moolah-slot.htmwas obviously taken from my page http://www.onlineslotsfinder.com/slots/mega-moolah.htm
FYI, andrewxr (CAP member) owns onlineslotmachines.biz.