New WordPress WP Query using posts from certain categories

There are some errors in your code, you have to change:

• $the_query -> have_posts() to $the_query->have_posts()
• $the_query -> the_post() to $the_query->the_post()
• WP_Query array(...) to WP_Query(array(...))
• 'showposts=6' to 'posts_per_page' => 6

Try this instead

$the_query = new WP_Query( array( 'category_name' => 'portfolio', 'posts_per_page' => 6 ) ); 
while ($the_query->have_posts()) : $the_query->the_post(); 
    the_post_thumbnail(); 
endwhile;