DummyImageFormatter.php

<?php

namespace Drupal\image_module_test\Plugin\Field\FieldFormatter;

use Drupal\Core\Field\Attribute\FieldFormatter;
use Drupal\Core\Field\FieldItemListInterface;
use Drupal\Core\Field\FormatterBase;
use Drupal\Core\StringTranslation\TranslatableMarkup;

/**
 * Plugin implementation of the Dummy image formatter.
 */
class DummyImageFormatter extends FormatterBase {
    
    /**
     * {@inheritdoc}
     */
    public function viewElements(FieldItemListInterface $items, $langcode) {
        return [
            [
                '#markup' => 'Dummy',
            ],
        ];
    }

}