Your daughter is becoming more aware of distance and what goes with it, and she thinks about it a lot more than she used to. She doesn't really understand it, so her emotions speak for her.
I do like the picture idea. You can remind her that when she misses other people, they might miss her, too. Help her write letters (yes, the old-fashioned kind). They're so unusual these days that everybody enjoys receiving one.
You are also doing well to talk about these people. "Isn't So-and-So wonderful? I'm so thankful that she belongs to us as a friend." If you can talk about the people and not just about the emotions, you'll be going in the right direction.
My children all went through crying periods when I would leave, too. It's the brains at work again - they start wondering about the great unknown of absence. Keep reassuring her. I think you're on the right track.
Thank you for NOT saying, "Oh, don't be silly!"