You know when you were young and you had these puzzles where there were two pictures, identical, except for eight differences, and you have to find the differences and circle them, well, that's a great way of making students talk to each other. The diffference, of course is that each student mustn't be able to see the other student's piece of paper.
Arrange the class into pairs, and get them to sit back to back with their partner. Then, give out the two pictures, one to each student. They must either describe their picture to the other person and isolate the differences, or ask questions about their partner's picture until a difference if found. This is a great way of practising asking questions, asking for the confirmation of ideas and speaking clearly. The only thing to think about is vocabulary. As they will invariably have to describe everything in the picture, it may be an idea to pre-teach some of the vocabulary that will come up.
The length of time the activity will take depends on level of detail in the pictures ("I've got 2396 leaves on the third tree in my picture; how many have you got?") and the number of differences you decide to include.