As a bonus there is some configuration for the top-left logo in the backend, the logo above the backend login form and extra CSS for the login form (which is far from perfect, but enough for a demonstration).

$relativePath = \TYPO3\CMS\Core\Utility\ExtensionManagementUtility::extRelPath($_EXTKEY);
$GLOBALS['TBE_STYLES']['logo_login'] = $relativePath . 'Resources/Public/Backend/Images/LogoLogin.png';
$GLOBALS['TBE_STYLES']['logo'] = $relativePath . 'Resources/Public/Backend/Images/typo3-topbar@2x.png';   
$GLOBALS['TBE_STYLES']['inDocStyles_TBEstyle'] .= '
  #t3-login-form:after {
    content: "\00a0";
    background-image: url("' . $relativePath . 'Resources/Public/Backend/Images/peeking.png");
    width: 220px;
    height: 187px;
    display: inline-block;
    overflow: hidden;
    position: relative;
    left: 201px;
    bottom: 533px;
  }
';