One of the strengths of Flickr is being able to add notes and comment to an image. Here is an example of an image that has lots of notes and comments discussing the image. Notes are used for talking about a small part of the image, comments are used for discussing the image as a whole.
This Flickr photo also tells you about some other things you can do with Flickr in class.
Preparation
Create a Flickr account for the class. Upload an ambiguous photo.
Instructions
Ask them go to Flickr and log in using the account you have set up. When pupils first login they will see a page that includes 'Your Photostream'. This will show the image you have uploaded.
Ask pupils to add their views about what is happening, or suggest newspaper headlines and taglines to the image. If there are people in the image, they could add notes suggesting what the person is saying.
Alternatives
You could add a complex image (perhaps a still of a crowd scene from a film) and get pupils to add notes about objects or facial expressions they find interesting.