From your top right "My Trulia" dropdown, choose "Listings". From your list of properties, directly under the photo, click "Edit Listing Details". From there, choose "Listing Photos". You will get a page asking to "Browse" for photos. Locate the photo you want to upload (you can upload more than one at a time, but that takes awhile on Trulia and can confuse). Click "Upload" and you should have a photo on your listing. Add additional photos and then reorder them to your heart's content.
Hope this helps. Best of luck selling your house! :-) ~~ Bobby
This brings up a good point about ownership of the information, agency, and responsibility to consumers. Hopefully Trulia will get on this and help you to get it updated. Not the first time this has happened!
Best wishes Chris;