Phew!
After 2 days, finally found it: it was in content-single.php. I added a “//” juste before “the_post_thumbnail” to prevent the displaying of thumbnail image in the second part of this piece of code :
<?php if ( 'image' != get_post_format() && 'gallery' != get_post_format() && 'video' != get_post_format() ) : ?>
<div class="entry-thumbnail">
<a href="<?php the_permalink(); ?>" title="<?php echo esc_attr( sprintf( __( 'Permalink to %s', 'skylark' ), the_title_attribute( 'echo=0' ) ) ); ?>" rel="bookmark"><?php the_post_thumbnail( 'skylark-single', array( 'class' => 'skylark-blog-thumbnail', 'alt' => get_the_title(), 'title' => get_the_title() ) ); ?></a>
</div>
<?php else : ?>
<div class="entry-thumbnail">
<a href="<?php the_permalink(); ?>" title="<?php echo esc_attr( sprintf( __( 'Permalink to %s', 'skylark' ), the_title_attribute( 'echo=0' ) ) ); ?>" rel="bookmark"><?php //the_post_thumbnail( 'skylark-single-full', array( 'class' => 'skylark-blog-thumbnail', 'alt' => get_the_title(), 'title' => get_the_title() ) ); ?></a>
</div>
<?php endif; ?>
