Show thumbnail on single posts

Add this code to functions.php (use custom child theme or plugin FunctionsPHP):

// add single post thumbnail
function myslug_add_single_post_thumbnail(){
	the_post_thumbnail( 'medium', array('class'=>'thumbnail alignright') );
}
add_action( 'basic_before_single_content', 'myslug_add_single_post_thumbnail' );

You can change medium in second string to another size:

• thumbnail – Thumbnail (default 150px x 150px max)
• medium – Medium resolution (default 300px x 300px max)
• large – Large resolution (default 640px x 640px max)
• full – Original image resolution (unmodified)