When is it PD? You can't always presume that content prepared under the direction of the federal government is in the public domain. FYI, a 1999 study determined that 15% of the materials offered by government agencies was under copyright. Only a "work of the U.S. government" is in the public domain. That's any work prepared by a federal officer or employee within the course of their duties. The federal government enters into contracts with many independent contractors (non federal employees) and in those cases, the contractor may retain rights under certain situations. Often, the government requires that the material be assigned to the U.S. government under federal acquisition regulations, and it is then, in turn, made PD. This seems to be the case with the Oak Ridge National Laboratory (ORNL) which is managed by UT–Battelle, LLC, a limited liability partnership between the University of Tennessee and Battelle Memorial Institute. In other words, many non-federal employees are contributing to the content at ORNL. The notice at the ORNL site states:
Copyright Status. Documents provided from the web server were sponsored by the U.S. Government under contract DE-AC05-00OR22725 with UT-Battelle, LLC, which manages the Oak Ridge National Laboratory. Accordingly, the U.S. Government retains a nonexclusive, royalty-free license to publish or reproduce these documents, or to allow others to do so, for U.S. Government purposes. Unless otherwise noted, they have been placed in the public domain, although we request the following credit line be used when documents or figures are used elsewhere: “Courtesy of Oak Ridge National Laboratory, U.S. Dept. of Energy."Although this notice is not a model of clarity -- what does it mean to use the material "elsewhere"? -- it does establish that material at the site is public domain unless noted otherwise. We're not sure whether that applies to the Flickr photostream, supposedly protected under Creative Commons. It is possible that the photos posted there by the Oak Ridge Lab News are protected separately and not subject to the government contract (in which case the only way to be certain is to contact ORNL). If, however, you can find the same photos at the ORNL site and there is no copyright limitation there, you can assume the pictures are PD.
Finally, as to including the notice, “Courtesy of Oak Ridge National Laboratory, U.S. Dept. of Energy," there is no obligation to provide attribution on a public domain photo and there is no indication that you have entered into a licensing arrangement with ORNL to do so. However, a statute does require that if public domain works produced by the federal government are reproduced that some notice should be given as to what is PD.