<?php 
$image = wp_get_attachment_image_src( get_post_thumbnail_id( $post->ID ), 'thumbnail' ); 

if ($image) : ?>
    <img src="<?php echo $image[0]; ?>" alt="" />
<?php endif; ?>

http://wordpress.stackexchange.com/questions/29881/stop-wordpress-from-hardcoding-img-width-and-height-attributes

This is an example of omitting the width and height attributes from a thumbnail img tag in Wordpress. Useful if you are working on a responsive design. Use inside The Loop.

Wordpress: Omit width/height attributes from img tag

wordpress, images

PHP