Heavy on the science geekery, this article looks at strict comparative analysis of all known (and considered) portraits of Shakespaere, most notably the Cobbe, Janssen and Droeshout. Includes a pretty cool montage of Droeshout and Cobbe.
Their conclusion, and I have to admit I have not fully understood the details of how they arrived at it, is
…that this clearly indicates once again that, only pending further research into its early history, can the Janssen portrait be admitted to the select company of genuine Shakespeare portraits; and that it cannot in all possibility be a copy of the Cobbe portrait. On the contrary, Janssen may have served as the model for the Cobbe.