The system's phpthumb integration is protected and only works when logged into the manager.
Outputting a link like /revolution/connectors/system/phpthumb.php?h=250w=250&src=/revolution/assets/01895_smellofholland_3840x1200.jpg&sx=85&sy=2&sw=1413&sh=1059&ipW=800&ipH=600 does not show an image when not logged in.
I would suggest using phpthumbof. It'll work when not logged in as well, plus it caches the images for you as well.