Change theme’s thumbnail to cropped WP featured image

Check out get_the_post_thumbnail()

$image_thumb = get_the_post_thumbnail( get_the_ID(), 'thumbnail');

Here’s a more specific example:

while (have_posts()) : the_post();

    echo '<article class="search-entry clearfix">';

    $image_thumb = get_the_post_thumbnail( get_the_ID(), 'thumbnail');

    echo ( $image_thumb ) ? sprintf('<a href="https://wordpress.stackexchange.com/questions/168885/%s" title="https://wordpress.stackexchange.com/questions/168885/%s" class="search-portfolio-thumb">%s</a>', get_permalink(), get_the_title(), $image_thumb) : '';