This will be subjective, of course, and I think that you can get different takes on the "lasting image" out of the book. For me, it is the image of Melinda imagining herself planted as a tree. I find this significant and lasting for a couple of reasons. The first would be that this aspect of regeneration from the Earth, something organic and pure, growing and emerging is representative of how Melinda undergoes growth and change throughout the narrative. Her growth into her own voice is represented by the trees, something that she had worked on in her art class. This is a significant image because it shows maturation and change in Melinda. From where she was at the start of the story to where she goes at the end of it, the image of the tree is a powerful one. The idea of a tree giving shelter to those who are in need is also representative of how Melinda warns Rachel about Andy and how she graffitis in the bathroom wall about him, inspiring others to do the same. This image is lasting because it is also representative of the last time Melinda views herself as outside a social order, for after the "planting" of herself, Melinda stands up to Andy and others follow her example. This is where the tree image becomes a lasting one for me in the story.