Imagem resolution responsive

You can use the timthumb for resize your images as you want.

if ( has_post_thumbnail() ) {
                    // get the src of the large size featured image
                    $src = wp_get_attachment_image_src( get_post_thumbnail_id($post->ID), 'large' );
                    $thumbnailSrc = $src[0];
                    // output image resized with timthumb 
                    ?>
                    <a href="https://wordpress.stackexchange.com/questions/97632/<?php echo get_post_meta($postid,"post_link', true) ?>" target="_blank">
                    <img src="<?php bloginfo('template_directory'); ?>/js/timthumb.php?src=<?php echo $thumbnailSrc; ?>&h=240&w=245" alt="">
                    </a>
                    <?php } ?>

And download that file timthumb

Hopefully your problem solve.