I must admit I found this on another website and think it is a very interesting exercise.
The latest post in the thread will have the phrase TIBM, (The Image Below Me) followed by a subject or theme.
So one replies to the thread with an image appropriate to the nominated subject.
Then create a new theme with TIBM.
So for example:
TIBM is cloudy
(so the next image should be in the theme of cloudy)