That code worked great!
I modified slightly below to get a dash between First_Name and FileName. (resulting in LastNameFirstName_image.jpg). And further added dash between Last_Name and First_Name after several attempts. (resulting in LastName_FirstName_image.jpg).
'FileName' => "".((isset($_POST["Last_Name"]))?$_POST["Last_Name"]:"") .""."_".((isset($_POST["First_Name"]))?$_POST["First_Name"]:"") ."_[FileName]",
thanks so much Jason!