Use the TreeView.ExpandImageUrl property to specify a custom image for the expandable node indicator. This image is displayed next to an expandable node and is usually a plus sign (+).

When the TreeView.ShowExpandCollapse property is set to true, the System.Web.UI.WebControls.TreeView control displays a plus sign (+) or minus sign (-) or a placeholder image next to a node to indicate whether the node is expandable, collapsible, or non-expandable, respectively. You can specify custom images by first setting the TreeView.ImageSet property to TreeViewImageSet.Custom, and then setting the TreeView.NoExpandImageUrl, TreeView.ExpandImageUrl, and TreeView.CollapseImageUrl properties to the URLs for the custom images. If no images are specified, the default images are used. To hide the expansion node indicators, set the TreeView.ShowExpandCollapse property to false.

To display a custom image correctly, you may need to set TreeView.ShowLines to false.

When using a custom image for the expandable node indicator, you should also consider setting the TreeView.ExpandImageToolTip property to specify a ToolTip for the image.

The value of this property is stored in view state.