Here are two examples of cards that are interesting both in front and back. The card on top is from a postcrosser living in Kiev. I love the way he played with the farm produce image in the front by including a whole block of stamps in the back with a farm family theme. The images were self-explanatory - no words needed!
The card below is from friend Sherri in Billings, MT. She used a postcard by Annie Sprinkle and Beth Stephens and then altered it. The words speak to expecations of marriage, I believe. The back of the card is filled with seal prints from a chop Sherri got made in Hong Kong. I love the beautiful design these chop marks bring to the card.