mrm_be/Configuration/TCA/Overrides/custom_components/contentimage.php

<?php

\TYPO3\CMS\Core\Utility\ExtensionManagementUtility::addTcaSelectItem(
 'tt_content',
 'CType',
  [
    'LLL:EXT:mrm_be/Resources/Private/Language/locallang_ttc.xlf:mrm_be_cm_contentimage.title',
    'mrm_be_cm_contentimage',
    'mrm_be_cm_contentimage_icon',
  ]
);

$GLOBALS['TCA']['tt_content']['columns']['pi_flexform']['config']['ds'][',mrm_be_cm_contentimage'] = 'FILE:EXT:mrm_be/Configuration/FlexForms/contentimage.xml';
$GLOBALS['TCA']['tt_content']['types']['mrm_be_cm_contentimage'] = [
  'showitem' => 'CType;;;1-1-1,pi_flexform;LLL:EXT:mrm_be/Resources/Private/Language/locallang_ttc.xlf:mrm_be_cm_contentimage.title,',
];