public function imageUrl($field = null)
    {
        $this->uploadFieldName = $this->getUploadFieldName($field);
        $this->uploadFieldOptions = $this->getUploadFieldOptions($this->uploadFieldName);

        // get the model attribute value
        $attributeValue = $this->getOriginal($this->uploadFieldName);

        // check for placeholder defined in option
        $placeholderImage = array_get($this->uploadFieldOptions, 'placeholder');

        return (empty($attributeValue) && $placeholderImage)
            ? $placeholderImage
            : $this->getStorageDisk()->url($attributeValue);
    }