Some people over think SEO. But I understand why. All search engine optimization is basically composed of made up rules by algorithm engineers. So it is something that is not really intuitive. You have to learn about it and over time you can make a better guess at what things matter for rankings and what do not.
I have heard the question on stock photos and SEO many times. Apparently, this topic made it all the way to Google. In this video, Matt Cutt’s comments on the question, “Do Stock Photos Hurt SEO?” Short answer is no. The long answer is in the video. While that is the case, Cutt’s does say that this could be something they consider in the future.