Successfully complete Campaign mode under the Insane difficulty setting to unlock a Gamerpic of Marcus.
Successfully complete Seriously 2.0 to unlock another Marcus Gamerpic.
Get the "Friends With Benefits" achievement to unlock a Gamerpic of Marcus and Dom.
With the Collectors Edition of the game, insert the bonus disc in and go to "Gamer Pictures". You will unlock both of the sets. One set has COG and the other has Locust.