Why does Abigail leave Susie's picture in the airport in Chicago?

She leaves Susie’s photograph behind as a symbol of her transition out of the state of trauma and grief, paralleling Jack’s heart attack as a way of him transitioning out of that state. You can read more about the power of photographs in this book at the GradeSaver link below,

http://www.gradesaver.com/the-lovely-bones/study-guide/section6/