// 图片alt强制标题
function image_alt($c) {
    global $post; // 全局量
    $title = $post->post_title; // 文章标题
    $s = array('/src="(.+?.(jpg|jepg|png|gif|bmp))"/i' => 'src="$1" alt="'.$title.'"');
    foreach($s as $p => $r){
    $c = preg_replace($p,$r,$c);
}
return $c;
}
add_filter( 'the_content', 'image_alt' );

代码来自:http://meiri.me/9047.html