I'm having the exact same problem. I am running Jetpack, but I do not have Photon enabled. Any other suggestions??
EDIT: It may not be the EXACT same, but it's hard to say. When I clear my cache the photos show up. But if I come back to the site the next day the images again display as broken links. I can't very well have our users refresh their cache on each visit! :-)
My site is at: http://www.faultlinetheatre.org/ . The 2nd and 4th box are the UPWs in question.
I've tested on Chrome, Firefox and Safari, on my Mac, iPad and Android all with the same results. I'm running WordPress 3.5.1.
I otherwise love this plugin! I really don't want to have to look elsewhere.