I work in a government department and our section (reasonably diverse with regard to age, sex, ethnicity etc.) were hauled out of our office to pose for a group photograph for some initiative or other. Don't know whether it ever made the public domain, though.
Another possibility is that they use picture agencies, such as Corbis, Getty or Hulton where you can get pictures of anything and anybody (although they have to pay for the use of these pix) but it saves having to do the donkeywork. The people in the agency photographs will already have given their permission so no further paperwork is necessary.