The idea has the same principle as the classic game of I Spy.
A photo is posted and the next person posts their own picture of something that they see in the previous photo. Of course follow up with "I spy ... (whatever it is you want to refer to)"
It could be an object in the photo you see, a color, a flower, animal, food, etc.
Example: someone posts a picture of a couple at a picnic.
The next person might see some cheese by the picnic basket, so they post a picture of some cheese.
Then the next person might see a sharp knife by the cheese picture and decide to post a picture of a ninja or something.
Just remember to state whatever it is you spy in the last photo.
Should be fun.
What do you spy in this photo?
I spy a turtle...
I spy a witch
I spy a monkey
I spy a castle
I spy a backpack
I spy mountain
I spy a cherry
I spy sharpie
I spy purple mountains