Have them place the cards around the circle where each postcard touches another. Ask them to explain why they made each connection. Encourage them to think of all the connections that are possible, even those not previously considered. Ask them to write their lists on the index cards. With younger students, this can be done as a disucssion with an adult writing their responses on 3x5 cards, placing the cards under each matched set.
Older students can write their own responses for their connections.
Pass out new index cards of a different color to the groups. Ask them to make a list on the new index cards describing why they think the previous group made those links and any other reasons they can think of. Have your students return to their original groups of cards to contrast and compare the two sets of lists. Have each person in the group select two cards that are connected. Pass out another color of index card and ask them to list all the ways that the two images are different.