I tried searching but I couldn't find anything adding an image for the current node. I came up with this code which will add an image to the current node. I bet someone is going to responsd with an easier way to do this too lol.
Protected Sub RadMenu1_NodeCreated(ByVal sender As Object, ByVal e As Telerik.Web.UI.RadTreeNodeEventArgs) Handles RadMenu1.NodeCreated
If Request.Url.AbsolutePath.Contains(e.Node.NavigateUrl.Remove(0, 1)) Then
e.Node.ImageUrl =
"images/yourimage.jpg"
End If
End Sub