Bagaimana cara Mengganti watermark TXT menjadi IMAGE/PNG

Assalamualaikum Warahmatullahi Wabarakatuh... bang robi, bang Bukhori dan mastah lainnya saya mau tanya 
saya sudah mengubah script untuk mengubah watermark txt ke iimag
 
 
	function tambah_listberita(){
		cek_session_akses('listberita',$this->session->id_session);
		if (isset($_POST['submit'])){
			$config['upload_path'] = 'asset/foto_berita/';
	        $config['allowed_types'] = 'gif|jpg|png|JPG|JPEG';
	        $config['max_size'] = '3000'; // kb
	        $this->load->library('upload', $config);
	        $this->upload->do_upload('k');
	        $hasil=$this->upload->data();
 
            $config['wm_overlay_path'] = 'public_html/logo.png';  // Watermark Image Source
            $config['wm_type'] = 'overlay';
            $config['width'] = '50';
            $config['height'] = '50';
            $config['padding'] = '50';
            $config['wm_opacity'] = '50';
            $config['wm_vrt_alignment'] = 'bottom';
            $config['wm_hor_alignment'] = 'right';
            $config['wm_vrt_offset'] = '100';
           $this->load->library('image_lib',$config);
            $this->image_lib->watermark();
 
tapi belum berhasil, petunjuk mastah,