Two themes are clearly seen here. One, it is the positive image people create in remembrance of a departed person. The persona wants to remember her grandma as how she was in the photo, not the days before she died. When our loved ones die, we no longer see the ugly side of a person, whether their physical appearance before dying (aging, diseased,bad plastic surgery), their horrible attitude or bad habits (substance abuse). We will remember them in their prime age, the great things about them and their contribution to society. Look at how all those hated, heavily criticised drug abusing, trash-talking, plastic surgery-loving celebrities are being revered and paid homage to only when in death. We forget and forgive all their shortcomings, unfortunately, only when they die.
Another theme is the coping with grief and loss. And one way to cope with grief and loss is to hang on to the good memories of a person, instead of remembering the painful way he or she died. The persona who was struck with grief by the loss of her grandma found a way to cope with it - which is to keep it in her heart that although her grandma is not with her anymore physically, she is still alive in her heart. This is probably the best way to deal with loss, by telling ourselves that even though that person is not with us anymore, he or she will always be with us in spirit. Believing in it brings comfort and closure.