get_the_post_thumbnail()
returns the full html output of the image, including the image <img />
tags. You want just the image src. Try this:
<div class="leftBox">
<?php
$query = new WP_Query( 'pagename=jour' );
// The Loop
if ( $query && $query->have_posts() ) :
while ( $query->have_posts() ) : $query->the_post();
if ( has_post_thumbnail() ) {
$img_src = wp_get_attachment_image_url( get_post_thumbnail_id( get_the_ID() ), 'thumbnail' );
?>
<div class="entry-content" style="background: url('<?php echo esc_url( $img_src ); ?>') no-repeat;">
<a href="https://wordpress.stackexchange.com/questions/223955/<?php echo get_permalink(); ?>" title="Go to <?php echo the_title(); ?>"><p><?php the_title(); ?></p></a>
</div>
<?php
}
endwhile;
endif;
/* Restore original Post Data */
wp_reset_postdata();
?>
</div>