It really depends on what age level your are dealing with and what type of picture.
For example if they are fairly high level, I would show a picture of "The Mona Lisa" and tell them that she is famous for her smile. Have them write- a story which explains why she sat for that picture with a smile in her face.
Show the picture "The Scream" by Munch, and have your pupils write why they think the person is screaming.
You can show 2 pictures and have your pupils compare them using comparatives and superlatives.
You can have pupils write what happens next after looking at a picture.
Show them an old picture of people and ask them how their life was different from ours? etc.
It all depends on what picture you choose and what your aim is.
Hope this helps.