The answers by the other professionals on here have covered most solutions. Your agent who has the listing needs to log into their MLS and update the pictures for you. MLS automatically syncs with several real estate sites, but usually takes 24 hours to update, every evening around midnight I believe. Your agent's company website most likely feeds from MLS. After MLS is updated all the other sites should be current. If there was an ad placed directly on a real estate website, you may have to amend the ad manually on the specific website to override it. Hope this helps!
You should first have the agent change the pictures in the MLS system. Those changes should syndicate out to dozens of other real estate websites that rely on MLS fed data. Some websites like Trulia, Zillow, Realtor.com, etc. will also allow the agent to make some changes to the listing on those individual sites which would be quicker than waiting on MLS syndication. Hope this helps.
Keller Williams Realty