This is a Metal Gear Solid 2 achievement.
This can only be done at the end of the Tanker chapter. When the captain is giving his speech, sneak past the soldiers and try to look at them from a safe position. You should spot some soldiers wearing just boxer shorts. Equip your camera and take a picture of one of them. When you reach the terminal at which you can upload the Metal Gear Ray photos to Otacon, he'll comment about the photo and you'll earn your achievement.
Alternatively, on every pair number of Tanker playthroughs (2nd playthrough, 4th playthrough), all guards on Hold 3 appear in their underwear. Take a photo of them and upload it to Otacon in the terminal.
You must align the camera properly to make it work. You'll know it worked the moment you take the picture, because Snake will comment something. Try taking pictures from different positions until Snake mutters something to himself.