backend/views/image/gallery.php

<?php
use \kartik\file\FileInput;
use yii\widgets\ActiveForm;
use yii\helpers\Html;
/* @var $this yii\web\View */
$this->title = 'Image gallery';
?>
<?php foreach ($files as $file): ?>
<img height="500px" src="<?= Html::encode($file) ?>">
<?php endforeach ?>