Featured image not showing

When changing your active theme, I would advise that you regenerate your thumbnails, this may resolve the issue you’re having.

Force Regenerate Thumbnails

This actually looks like a deliberate design decision by your theme’s author; I see the same thing on the theme’s demo site. You could fix this by making a child theme and then copying this PHP into your child theme’s functions.php:

function basic_child_thumbnail() {
	$thumbnail_size = apply_filters( 'basic_singular_thumbnail_size', 'medium' );
	$attributes = apply_filters( 'basic_singular_thumbnail_attr', array('class'=>'thumbnail') );

	echo '<a href="<?php the_permalink() ?>" title="<?php the_title_attribute(); ?>" class="anons-thumbnail">';
	the_post_thumbnail( $thumbnail_size, $attributes );
	echo '</a>';
}
add_action( 'basic_before_single_content', 'basic_child_thumbnail' );