I found this picture of Pic Dawson and I have to agree with Richard, it isn't him in the picture with Cass. Malevolent looking though, isn't he?
http://tatelabianca.blogspot.com/2005/1 ... awson.html
Also, I realized that the picture in question (Cass' head on John Doe's lap) was one of the pictures in the "images" section of this website! On closer inspection, it does look like Donovan, who we know had a very close relationship with Cass. The mystery thickens...