The ID of the WebImageViewer used to display the selected thumbnail.
You can connect a WebImageViewer control to the WebThumbnailViewer control by setting this property to the required Viewer ID. After that any changes to the WebImageViewer's image will be reflected automatically in the selected thumbnail and any selected thumbnail's image will be loaded into the WebImageViewer control.
If this property is not set to a WebImageViewer ID you have to update the WebImageViewer control manually if the SelectedIndex property changes. Do this by handling the javascript version of the SelectedIndexChanged event and calling the javascript version of the OpenImageUrl(String) method. Also you will be responsible for updating the selected thumbnail image, if the image loaded in the WebImageViewer changes by an image processing command being applied to it. Do that by handling the javascript event WebImageViewer.ImageChanged and calling the javascript method Update and passing the ID of the WebImageViewer that has had its image changed.
For more information refer to the tutorial, Updating WebControls Manually