As an agent, I've had to ask the same question of Trulia and below is what I found:
The MRIS (our local MLS service) no longer uploads directly to Trulia. They stopped their relationship about 2 years ago. Zillow also no longer has a relationship with MRIS as of January this year. (Zillow is typically 2 months to 2 years out of date on most of their information anyway, so don't believe most of what you find there)
If your brokerage does not have an automatic feed to Trulia (a few of the larger nat'l companies do through their franchise) then your agent has two choices. Upload directly (with this option they can actually put more photos in than the MLS allows) or, if they have a Trulia account, then they can call customer service and have them add it from Realtor.com on request.