Post two photos at a time taken at the same location, but years, months, days, even hours apart.
Alternatively, comparisons of things such as buses, police cars, fire trucks, phone books, library cards, etc. are also welcome - photos of this type don't have to be taken in the same location but of the old and new thing.
Post your NOW photo first and your THEN photo second.
If you don't post two photos (or one photo showing two distinct time periods), your photo will be removed from the pool.
Alternatively, you can post one shot and another Flickr member can post the other, as long as the NOW shot is posted first, and there are a total of two photos for the set!