/**
 * @inheritdoc
 */
public function behaviors()
{
    return [
        'imageUploaderBehavior' => [
            'class' => 'demi\image\ImageUploaderBehavior',
            'imageConfig' => [
                // Name of image attribute where the image will be stored
                'imageAttribute' => 'image',
                // Yii-alias to dir where will be stored subdirectories with images
                'savePathAlias' => '@frontend/web/images/products',
                // Yii-alias to root project dir, relative path to the image will exclude this part of the full path
                'rootPathAlias' => '@frontend/web',
                // Name of default image. Image placed to: webrooot/images/{noImageBaseName}
                // You must create all noimage files: noimage.jpg, medium_noimage.jpg, small_noimage.jpg, etc.
                'noImageBaseName' => 'noimage.jpg',
                // List of thumbnails sizes.
                // Format: [prefix=>max_width]
                // Thumbnails height calculated proportionally automatically
                // Prefix '' is special, it determines the max width of the main image
                'imageSizes' => [
                    '' => 1000,
                    'medium_' => 270,
                    'small_' => 70,
                    'my_custom_size' => 25,
                ],
                // This params will be passed to \yii\validators\ImageValidator
                'imageValidatorParams' => [
                    'minWidth' => 400,
                    'minHeight' => 300,
                ],
                // Cropper config
                'aspectRatio' => 4 / 3, // or 16/9(wide) or 1/1(square) or any other ratio. Null - free ratio
                // default config
                'imageRequire' => false,
                'fileTypes' => 'jpg,jpeg,gif,png',
                'maxFileSize' => 10485760, // 10mb
                // If backend is located on a subdomain 'admin.', and images are uploaded to a directory
                // located in the frontend, you can set this param and then getImageSrc() will be return
                // path to image without subdomain part even in backend part
                'backendSubdomain' => 'admin.',
            ],
        ],
    ];
}

/**
 * @method getImageSrc($size = null)
 * @property string $imageSrc
 */

// big(mainly) image
Html::img($model->getImageSrc());
// or shortly
Html::img($model->imageSrc);

// medium image
Html::img($model->getImageSrc('medium_'));

// small image
Html::img($model->getImageSrc('small_'));

// image size "my_custom_size"
Html::img($model->getImageSrc('my_custom_size'));

<?php $form = ActiveForm::begin([
    'options' => ['enctype' => 'multipart/form-data'],
]); ?>

<?= $form->field($model, 'image')->widget('demi\image\FormImageWidget', [
    'imageSrc' => $model->getImageSrc('medium_'),
    'deleteUrl' => ['deleteImage', 'id' => $model->getPrimaryKey()],
    'cropUrl' => ['cropImage', 'id' => $model->getPrimaryKey()],
    // cropper options https://github.com/fengyuanchen/cropper/blob/master/README.md#options
    'cropPluginOptions' => [],
]) ?>

/**
 * @inheritdoc
 */
public function actions()
{
    return [
        'deleteImage' => [
            'class' => 'demi\image\DeleteImageAction',
            'modelClass' => Post::className(),
            'canDelete' => function ($model) {
                    /* @var $model \yii\db\ActiveRecord */
                    return $model->user_id == Yii::$app->user->id;
                },
            'redirectUrl' => function ($model) {
                    /* @var $model \yii\db\ActiveRecord */
                    // triggered on !Yii::$app->request->isAjax, else will be returned JSON: {status: "success"}
                    return ['post/view', 'id' => $model->primaryKey];
                },
            'afterDelete' => function ($model) {
                    /* @var $model \yii\db\ActiveRecord */
                    // You can customize response by this function, e.g. change response:
                    if (Yii::$app->request->isAjax) {
                        Yii::$app->response->getHeaders()->set('Vary', 'Accept');
                        Yii::$app->response = Response::FORMAT_JSON;

                        return ['status' => 'success', 'message' => 'Image deleted'];
                    } else {
                        return Yii::$app->response->redirect(['post/view', 'id' => $model->primaryKey]);
                    }
                },
        ],
        'cropImage' => [
            'class' => 'demi\image\CropImageAction',
            'modelClass' => Post::className(),
            'redirectUrl' => function ($model) {
                /* @var $model Post */
                // triggered on !Yii::$app->request->isAjax, else will be returned JSON: {status: "success"}
                return ['update', 'id' => $model->id];
            },
        ],
    ];
}