A script is used to search for any Challenge posts that contain that WIP tag just before the last posted map in that thread. The script scoops up the tagged image and becomes posted in the Challenge Thumbnail page.
The purpose is so viewers of the Challenge don't have to dig through each thread to see the maps involved in the challenge. They visit the thumbnail page which shows all the maps posted in the challenge so far. Clicking on the thumbnail will send the viewer to the particular page in thread to see the large sized version of the map thumbnail.
We do this as a service to those not in the challenge who want to see all the maps. This becomes especially useful when there are many entries for a given challenge. In September 08, the challenge had 30 participants which made it "bear" for anyone to try and see all the map posts.
Its really part of the challenge requirements that you place that "WIP tag" before the posting of a map thumbnail.