我在下面使用了这段代码，虽然有点多，但它基本上是通过帖子缩略图、附加图像、内容中的图像，然后循环到每个类别的图像（这是我网站上的固定设置，因此在get_post_category_image. 主要用途是通过最后一个函数echo get_post_image():

function get_post_content_thumbnail($num, $width=\'\', $height=\'\') {
    $content = get_the_content();
    $count = substr_count($content, \'<img\');
    $start = 0;
    for($i=1;$i<=$count;$i++) {
        $imgBeg = strpos($content, \'<img\', $start);
        $post = substr($content, $imgBeg);
        $imgEnd = strpos($post, \'>\');
        $postOutput = substr($post, 0, $imgEnd+1);
        $postOutput = preg_replace(\'/width="([0-9]*)" height="([0-9]*)"/\',
                                   \'\',
                                   $postOutput);
        $postOutput = str_replace(array(\'class="alignright"\', \'class="alignleft"\'), \'\', $postOutput);
        $postOutput = preg_replace(\'/title="([\\w-_]+)"/\',
                                   \'\',
                                   $postOutput);
        $image[$i] = $postOutput;
        $start=$imgEnd+1;
    }

    if(isset($image) && stristr($image[$num],\'<img\')) {
        return str_replace(\'<img \', \'<img \' . image_hwstring($width, $height), $image[$num]);
    }
}

function get_post_attachment_thumbnail($width=\'\', $height=\'\') {
    $files = get_children(\'post_parent=\'.get_the_ID().\'&post_type=attachment&post_mime_type=image\');
    if($files) {
        $keys = array_reverse(array_keys($files));
        $j=0;
        $attachment_id = $keys[$j];
        $image = image_downsize($attachment_id, \'thumbnail\');
        $image[0] = str_replace(\'http://yoast.com\', \'http://cdn.yoast.com\', $image[0]);
        return \'<img src="\' . $image[0] . \'" alt="" \' . image_hwstring($width, $height) . \'/>\';
    }
}

function get_post_category_image($p_id=null, $width=\'\', $height=\'\'){
    global $post_id, $top_category;
    if (!$p_id)
        $p_id = $post_id;

    if ( isset($top_category) )
        return $top_category;

    $post_categories = get_the_category( $p_id );

    $slug = count($post_categories) ? $post_categories[0]->slug : \'\';

    if ($slug && file_exists(TEMPLATEPATH . \'/img/post-img/\'.$slug.\'.png\')){
        return \'<img \' . image_hwstring($width, $height). \' src="\' . get_bloginfo(\'template_url\') . \'/img/post-img/\'.$slug.\'.png" alt="\'.$post_categories[0]->cat_name.\'"/>\';
    } else {
        return \'<img \' . image_hwstring($width, $height). \' src="\' . get_bloginfo(\'template_url\') . \'/img/post-img/default.jpg" alt=""/>\';
    }
}

function get_post_image($string_size=\'post-thumbnail\', $width=0){
    if ( has_post_thumbnail() ) :
        return get_the_post_thumbnail(NULL, $string_size, array(\'title\'=>\'\'));
    elseif( $attached_image = get_post_attachment_thumbnail($width)):
        return $attached_image;
    elseif( $post_content_thumbnail = get_post_content_thumbnail(1, $width)):
        return $post_content_thumbnail;
    else:
        return get_post_category_image(NULL, $width);
    endif;
}