The person who won the previous week's contest gets to select the Source Image that is to be used for the current week's contest.
If you are that winner, post anytime on Sunday, after you've been declared the winner, but no later than Monday or people may wonder about your whereabouts and start to send out search dogs.
Name the subject of your post as something that the other Walrus players will easily recognize. Something like what is outlined in 3.1. above.
Also, you need to post the Source Image at least twice; Sunday/Monday and Tuesday/Wednesday. This newsgroup can get a lot of traffic some weeks and that means posts can get bumped off the servers pretty quickly.