In the Options menu, you'll see "Photo Album" listed as one of the options. Getting access to this is a matter of taking photographs of specific characters in the game. After you finish the game, at the end of the credits a screen will come up giving you the "activation code" that turns those photos into hidden goodies.

Three of the four photos are of people involved in the story of the musical ghost. When you finish that story, the locations disappear, so you need to take the photos before you send Isaac to his rest.

The Musician

The four people you need to photograph are Joey (at some point when no one else is around), Isaac Brown, Dwayne from Jambalaya Records, and Cecil Sharpe.

At the end of the credits, watch for a screen telling you that the activation code is "realugly." (That code might not work until you've played the game through at least once, though.)

To access the hidden material, click on Photo Album, then click on one of the photos and enter the activation code. Joey's photo yields bloopers from the voice recording sessions; Isaac's gets you concept art for this game and the next game, Blackwell Convergence; Dwayne's gets you extra music tracks; and Cecil's photo gives commentary from Erin Robinson, the artist.