Pasting the url returned in the API results for a screenshot (e,g, http://www.webpagetest.org/results/13/07/29/ND/3BNM/1_screen.jpg) returns a 404 nginx error, but when navigating to the screenshot from the results, the page works - the url is the same. Did something just change? This worked fine last week.
I expect what you are seeing is the archiving code in action. Test results are archived off to slower storage after a day or two. There is code that will automatically restore the archived tests when any of the pages or API entry points are accessed but accessing the image directly won’t trigger the restore and will give you a 404 after the test has been archived (and should work again if any of the user-facing entry points for that test are accessed).