In the three holds as you are approaching RAY on the Tanker mission, you will see marines staring at Commandant Dolph, who is giving a speech in front of the new Metal Gear. In these crowds of marines you will come across a few with no pants on, just boxers. Snap a photo, then get to hold 3 (if you are not already there). Upload the picture on the terminal at the right of hold 3, Otacon's little avatar will laugh and this trophy will pop.
REMEMBER: You need to send this picture to Otacon for the trophy to pop.
Fun Facts about Scott Dolph:
*** Spoiler - click to reveal ***This character was named after a Konami Representative and, at one point, Hideo Kojima's personal translator. The real Scott Dolph is also a voice actor in the game - he is one of the hostages in B1 when you are looking for Ames.
Questions, corrections and neg vote justifications below.