First do you have the property linked to your account showing you as the listing agent? If you so you can just go to your profile and click on My Listings. This will take you to the listing dashboard. You can then click on the listing and the photos. Now since Trulia is actually a third party site it is fed by other websites. So the better way would be to go to the site that uploaded the listing to Trulia and change the pictures there. That way they will not be wiped out if you make another change on the regular listing and update it.
Hope this helps,... more