유튜브 썸네일의 경우 유튜브 아이디 추출이 필요해서 아래의 코드를 참고를 해보세요.

<?php
$tv_tot_count = 0;
$k = 0;
for ($i=1;$i<=$tv_idx;$i++)
{
    $tv_wr_idx = $tv_idx - ($i - 1);
    $tv_wr_id = get_session("ss_tv[$tv_wr_idx]");
    $tv_board = get_session("ss_tv_board[$tv_wr_idx]");
    $tv_board_prefix = $g5['write_prefix'].$tv_board;

    $rowx = sql_fetch(" select wr_id, wr_subject, wr_content from {$tv_board_prefix} where wr_id = '$tv_wr_id' ");
    if(!$rowx['wr_id'])
        continue;
    
    if ($tv_tot_count % $tv_div['img_length'] == 0) $k++;

    $wr_subject = get_text($rowx['wr_subject']);
    
    // 유튜브 영상 ID 추출
    $youtube_id = '';
    if (preg_match('/(?:https?:\/\/)?(?:www\.)?(?:youtube\.com|youtu\.be)\/(?:watch\?v=)?(.+)/', $rowx['wr_content'], $matches)) {
        $youtube_id = $matches[1];
    }

    if ($youtube_id) {
        // 유튜브 썸네일 URL 생성
        $thumb_url = "https://img.youtube.com/vi/{$youtube_id}/mqdefault.jpg";
        $img_content = '<img src="'.$thumb_url.'" alt="'.$wr_subject.'" width="'.$tv_div['img_width'].'" height="'.$tv_div['img_height'].'">';
    } else {
        $thumb = get_list_thumbnail($tv_board, $tv_wr_id, $tv_div['img_width'], $tv_div['img_height']);
        if($thumb['src']) {
            $img_content = '<img src="'.$thumb['src'].'" alt="'.$thumb['alt'].'" width="'.$tv_div['img_width'].'" height="'.$tv_div['img_height'].'">';
        } else {
            $img_content = '<span style="width:'.$tv_div['img_width'].'px;height:'.$tv_div['img_height'].'px"><img src="/logo.gif" width="70" height="50"></span>';
        }
    }

    if ($tv_tot_count == 0) echo '<ul id="stv_ul">'.PHP_EOL;
    echo '<li class="stv_item c'.$k.'">'.PHP_EOL;
    echo '<a href="'.G5_BBS_URL.'/board.php?bo_table='.$tv_board.'&wr_id='.$tv_wr_id.'">'.$img_content.'';
    echo '<br>';
    echo cut_str($wr_subject, 10, '</a>').PHP_EOL;
    echo '</li>'.PHP_EOL;

    $tv_tot_count++;
} 
if ($tv_tot_count > 0) echo '</ul>'.PHP_EOL;
?>