If a TreeNode does not have an image set, the image specified by the ImageKey property will be shown when the node is not selected. When the node is selected, the first item in the ImageList will be shown.
The ImageKey and ImageIndex properties are mutually exclusive, meaning if one is set, the other is ignored. If you set the ImageKey property, the ImageIndex property is automatically set to -1. Alternatively, if you set the ImageIndex property, the ImageKey is automatically set to an empty string ("").