What's a good Photoshop-type process to transform stock photos with identifiable faces into illustration or "art" with a more abstract/anonymous effect?

Asked by kevbo (25672) February 26th, 2015 from iPhone

I regularly use stock photos for projects at work, but I cannot use identifiable faces. I wonder if there are some good tricks to still use those kinds of photos while making the faces (and the entire image) more like illustration/art.

My Photshop skills are pretty limited at this point.