DummyImageFormatter.php

<?php

namespace Drupal\image_module_test\Plugin\Field\FieldFormatter;

use Drupal\Core\Field\FieldItemListInterface;
use Drupal\Core\Field\FormatterBase;

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

}