您可以使用wp_generate_attachment_metadata 过滤器：

function wpse_183835_to_jpeg( $meta, $post_id ) {
    $sizes_to_convert = array(
        \'thumbnail\',
    );

    $path = dirname( get_attached_file( $post_id ) );

    foreach ( $sizes_to_convert as $size ) {
        if ( ! empty( $meta[\'sizes\'][ $size ] ) ) {
            $data = $meta[\'sizes\'][ $size ];
            if ( $data[\'mime-type\'] === \'image/png\' && is_file( $file = "$path/{$data[\'file\']}" ) ) {
                if ( $image =@ imagecreatefrompng( $file ) ) {
                    // Change file extension
                    $file_jpg = preg_replace( \'/\\.[^\\.]+$/\', \'\', $data[\'file\'] ) . \'.jpg\';

                    if ( @ imagejpeg( $image, "$path/$file_jpg", 90 ) ) { // Save new jpg version
                        // Update metadata with new filename
                        $meta[\'sizes\'][ $size ][\'file\'] = $file_jpg;
                        // Delete png version
                        @ unlink( $file );
                    }

                    @ imagedestroy( $image );
                }
            }
        }
    }

    return $meta;
}

add_filter( \'wp_generate_attachment_metadata\', \'wpse_183835_to_jpeg\', 10, 2 );

https://codex.wordpress.org/Function_Reference/wp_generate_attachment_metadata