我用一个生成整个标记的新短代码来解决这个问题。像这样：

function generate_image_tag($atts) {
    $atts = shortcode_atts( array(
        \'img\' => \'\',
        \'title\' => \'\',
        \'theme_url\' => get_bloginfo(\'template_url\'),
    ), $atts );
    
    //File path details
    $file_info = pathinfo($atts[\'img\']);
    $file_dir = $file_info[\'dirname\'] != \'.\' ? $file_info[\'dirname\'].\'/\' : \'\';
    $file_url = $file_dir . $file_info[\'filename\'];
    
    $output = \'<picture>
                <source srcset="\'.$atts[\'theme_url\'].\'/images/\'.$file_url.\'.webp" type="image/webp">
                <img src="\'.$atts[\'theme_url\'].\'/images/\'.$atts[\'img\'].\'">
            </picture>\';
    
    return $output;
}
add_shortcode(\'generate_image\', \'generate_image_tag\');


[generate_image img="image.png" title="Title"]