The current winner posts an image and anybody who feels like it puts up an amusing caption to go with it. After an amount of time (usually depending on the popularity of the thread, one to three days at most), the winner selects their personal favourite submission. The user who posted the winning submission becomes, the new winner and the process is repeated. If the new winner fails to post an image within, say, a day then it's usually up to the previous winner to announce a runner up, who becomes the new new winner. The images should be selected with the Caption Contest in mind. Small groups of interesting looking people often work quite well. Inanimate objects don't work well at all.
I'll go first and select a winner to post the next image in a couple of days.
