Carl Zimmer tells us:
Facebook lets 3rd party advertisers use your posted photos without your permission. To opt out: Click on SETTINGS (located on top of page in blue bar, next to logout); Select PRIVACY SETTINGS; Select NEWS FEEDS and WALL; Select the TAB that reads Facebook Ads. There will be a drop down box; Select NO ONE. Save your changes & then pass this on. [Thanks to Virginia Postrel]
ADDED: You may need to turn off ad blocking to make this work.