You've already signed up for an account, so you're half-way there. Go to your "My Listings" page (I've included the link here). You'll find instructions there on how to search for your listings (if they're already on Trulia) or how to submit them (if they're not yet on Trulia).
In addition to the benefit of having your photo on your listing, you'll also ensure that any leads from consumers go straight to you. Additionally, you'll get a weekly email from us with traffic stats on your listing. If you'd like even more exposure for your listings, you can also sign up for Trulia Pro -- see http://www.trulia.com/truliapro
Hope this helps!