Charlie and Gerard are right on the money, as well. Essentially, if the listing is updated by the listing office, trulia, realtor.com, zillow, and many others will reflect accordingly. However, some of the websites will only show a few pictures, unless the realtor or the office they work for pays to optimize the listing.
I claim my listings on as many sites as I can, so I can ensure they display the best features of the home. In addition, your listing agent can pull up traffic reports and see how various advertising venues are paying off.