Here are step by step instructions to use Google's search by image function, from Slaw. Basically, go here and upload your image by clicking on the camera icon. Maybe someday soon if you somehow acquire a photo of a party or a witness, you will be able to run a search by image on it and find things posted on the internet under a user name. Right now, I don't think Google has its facial recognition software running with this seach.
Hat tip to Future Lawyer for the link.