Easy enough trophy - I would classify it as "damn near unmissable".
Beat the Tanker mission with Snake and you will unlock the Digital Camera for the Tanker.
You can also get the Digital Camera on the Plant mission in the distribution center (the strut with all the conveyor belts). Hop on on the conveyor, and get in a box. It will take you to another room where the digital camera is available.
Questions, corrections and neg vote justifications below.