How to get thumbnail with pure PHP in a WordPress database?

Okay, a couple of things – first I would suggest modifying your main query to grab the thumbnail ID at the same time (I have also changed it to only get posts that have a thumb):

$sql = "SELECT wp_posts.*, meta_value AS thumbnail_id FROM wp_posts " .
       "LEFT JOIN wp_postmeta ON wp_posts.ID = wp_postmeta.post_id " .
       "WHERE post_type="post" AND meta_key = '_thumbnail_id' AND meta_value != '' " .
       "ORDER BY post_date ASC";

Now for the processing:

while ( $linha = $stmt->fetch ( PDO::FETCH_OBJ ) ) {

    $meta = $pdo->prepare( 'SELECT meta_value FROM wp_postmeta WHERE meta_key = "_wp_attachment_metadata" AND post_id = ?' );
    $meta->execute( array( $linha->thumbnail_id ) );

    $file = null; // Failsafe in case anything below fails
    if ( $data = $meta->fetchColumn() ) {
        if ( $data =@ unserialize( $data ) ) {
            $file = $data['file'];

            if ( ! empty( $data['sizes'] ) ) {
                $sizes = $data['sizes'];

                if ( ! empty( $sizes['magento-image'] ) )
                    $file = $sizes['magento-image']['file'];
                elseif ( ! empty( $sizes['thumbnail'] ) ) // You might want to fallback, up to you
                    $file = $sizes['thumbnail']['file'];
            }

            if ( $file !== $data['file'] ) {
                // Files stored in sizes are only the basename
                // If you use year/month uploads, we need to grab that path

                if ( '.' !== $path = dirname( $data['file'] ) )
                    $file = "$path/$file";
            }
        }
    }

    if ( $file ) {
        // Gotcha!
        echo "<img src="http://example.com/wp-content/uploads/$file" />";
    }

}

This code is not tested, so see how you go and report back with any errors.