「ratio_3_2」でトップページ画像のalt設定方法

<div class="mainvisual">
<div class="container">
<div class="pop">
<div class="row">
<div class="mainvisual-col-01"><?php $this->Ratio32->get_main_image( array( 'num' => 1, 'type' => 'jumbotron', 'h' => 'h1' ) ); ?></div>
<div class="mainvisual-col-02"><?php $this->Ratio32->get_main_image( array( 'num' => 2, 'type' => 'thumbnail' ) ); ?></div>
<!-- /.row --></div>
<!-- /.pop --></div>
<!-- /.container --></div>
<!-- /.mainvisual --></div>
<div class="pickup">
<div class="container">
<div class="pop">
<div class="row">
<div class="col-md-4"><?php $this->Ratio32->get_main_image( array( 'num' => 3, 'type' => 'thumbnail', 'more' => true ) ); ?></div>
<div class="col-md-4"><?php $this->Ratio32->get_main_image( array( 'num' => 4, 'type' => 'thumbnail', 'more' => true ) ); ?></div>
<div class="col-md-4"><?php $this->Ratio32->get_main_image( array( 'num' => 5, 'type' => 'thumbnail', 'more' => true ) ); ?></div>
<!-- /.row --></div>
<!-- /.pop --></div>
<!-- /.container --></div>
<!-- /.pickup --></div>