Participants go to Flickr and search for "IMG_####". There are anywhere from a few hundred to a couple thousand results. One chooses an image or two from among the results posts it to his/her blog (along with attribution and a link back to the page where the image appears)--The blogger then writes about the image. The writer can make up a story about it, connect it to some issue, or just post it as an image he/she really likes.
By AnotherToughDayInAfricaSee more photos, or visit his profile.