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Use the TreeNodeStyle.ImageUrl property to specify a custom image for style. This image is displayed next to the node, and it can be in any image-file format (such as .jpg, .gif, .bmp, and so on), as long as the client browser supports that format.
If both the TreeNodeStyle.ImageUrl and TreeNode.ImageUrl properties are set, the image that is displayed is the one that is specified by the TreeNode.ImageUrl property.
The value of this property is stored in view state.