Simply post the URL (web address) of the Flickr page containing the nun photo into the comment's text. (Note: There should only be one photo on the Flickr page in question. If there is more than one photo shown, click your chosen photo to be taken to its unique page.) Oh, and don't forget to include your email address in the comments field provided, so I'll be able to contact you (it won't be published).
For the avoidance of doubt, there are no tricks involved. There is at least one photo of a real, live nun, who is very clearly a nun, and not just some indistinguishable speck/blob in the background.
Think of it as Where's Wally, but with nuns.
The judge's decision is final, no correspondence will be entered into, etc., etc.
Oh, is it empty already? Next bottle, I think.
Postscript: We have a winner. Congratualtions, Philip. The nun photo is here.