"Hey, Mark, Carrie, and Harrison. The Stormtroopers want to take a photo with you. I know you hate it, but let's just do it." That's what it looks like happened here. Mark Hamill and Harrison Ford were not into it, but Carrie decided to make the most of it, which is very typical of Carrie Fisher. I think you can see in this picture why she was known for being such a delight on set. She could make a dark room turn bright as the sun.
At the time this probably seemed like a photo just like any other these people might have taken. But for fans, this is a special moment to see. We probably all have pictures that are like this (sans the Storm Troopers), but ours don't carry the weight that this photo does. Star Wars truly is something special.