Hughes and Sivasailam have published a book called Photo Jolts!, which is available for purchase, along with photo cards, from various distributors.  We noticed that people who bought the book on Amazon often looked at sets of postcards after looking at this book. Our office owns a set of photos purchased from VisualsSpeak and another set of postcards purchased from Experiential Tools that we have found to be complementary to the Photo Jolts! exercises.