Citing Digitalbattle :

"Oh, Zynga, will thy never learn. The company which has been blatant in its ripping off other developers’ game mechanics has now been found to steal stock photos from iStockphoto. Specifically, the Zynga Poker game features a vector artwork of a football stadium, with the watermark still there. Our reader Anand did some digging and found the artwork on iStockphoto, from artist Todd Harrison."