This is a photo I snapshoted a minute before the sun went down. I think this represents serenity because usually when you think about sunset, it is a time when people finish working and settle back home. The mother nature turns back to serenity.
I think this picture is good demonstration of serenity because since babies are naive, they don’t have the expressions of stress or anxious like adults when they sleep. Therefore, when people see babies are asleep, what first comes into their mind is serenity. Picture from eppstein: http://www.flickr.com/photos/eppstein/2278819584/sizes/l/in/photostream/