try this
Protected Sub HTMLEditorExtender_ImageUploadComplete(sender As Object, e As AjaxControlToolkit.AjaxFileUploadEventArgs)
Dim fullpath As String = Server.MapPath("~/img/ar/") + e.FileName
HTMLEditorExtender.AjaxFileUpload.SaveAs(fullpath)
e.PostedUrl = Page.ResolveUrl(fullpath)
End Sub