TYPO3 Core

If you add an item to the "Contains Plugin" value of page the icon in TCEForms is only shown if the icon value is set.

But for the page tree rendering the sprite icon (if defined) is used to show an icon belonging to your selection.

$GLOBALS['TCA']['pages']['columns']['module']['config']['items'][] =
    array(
        'LLL:EXT:my_ext/Resources/Private/Language/locallang_db.xlf:contains-my_ext',
        'my_ext',
        \TYPO3\CMS\Core\Utility\ExtensionManagementUtility::extRelPath('my_ext') . 'ext_icon.png'
    );

\TYPO3\CMS\Backend\Sprite\SpriteManager::addTcaTypeIcon(
    'pages', 
    'contains-my_ext', 
    \TYPO3\CMS\Core\Utility\ExtensionManagementUtility::extRelPath('my_ext') . 'ext_icon.png'
);

This will show an icon in the page tree and in the backend form.

$GLOBALS['TCA']['pages']['columns']['module']['config']['items'][] =
    array(
        'LLL:EXT:my_ext/Resources/Private/Language/locallang_db.xlf:contains-my_ext',
        'my_ext'
    );

\TYPO3\CMS\Backend\Sprite\SpriteManager::addTcaTypeIcon(
    'pages', 
    'contains-my_ext', 
    \TYPO3\CMS\Core\Utility\ExtensionManagementUtility::extRelPath('my_ext') . 'ext_icon.png'
);

This will only show an icon in the page tree.