Dan from Trulia here. You can change your photo from your profile page (located at the URL I'm including here as a link): http://www.trulia.com/account/settings/
Just click on the Upload Photo button and use a photo of yourself (in practice, profile photos that are close to square in shape work best).
While you're on your profile page, be sure to update other information too. This way, potential home buyers or sellers who find you on Trulia can learn a bit more about you. Good luck!