First you will need the name and address, email or fax number of the site's designated DMCA (Digital Millennium Copyright Act of 1998) agent. In most cases this information has been provided in the sites Terms of Service
The name of the Instructables DMCA agent for instance is: Eric Wilhelm
1467 Park Ave
Emeryville, CA 94608
Normally the phone number which is provided for the DMCA agent is a fax number, but if it is not then you can send a certified letter to the snail mail address. Note that a picture is not necessary, although a likeness or avatar might help the site's staff connect the name with the person...