Responsive attachment image as background in WordPress. Working example on my photo-blog: http://olegs.be/gallery/turin-italy/

responsive-background-attachment-image-wordpress.txt

	<?php // Get attachment image URLs
		$thumb_full = wp_get_attachment_image_src( get_post_thumbnail_id($post->ID), 'full' );
		$thumb_large = wp_get_attachment_image_src( get_post_thumbnail_id($post->ID), 'large' );
		$thumb_medium = wp_get_attachment_image_src( get_post_thumbnail_id($post->ID), 'medium' );
		$thumb_url_full = $thumb_full['0'];
		$thumb_url_large = $thumb_large['0'];
		$thumb_url_medium = $thumb_medium['0'];
	?>
	
	<style>
		.gallery-cover {
			background-image: url( <?php echo $thumb_url_medium; ?> );
		}
		
		@media all and (min-width: 768px) {
			.gallery-cover {
				background-image: url( <?php echo $thumb_url_large; ?> );
			}
		}
		
		@media all and (min-width: 992px) {
			.gallery-cover {
				background-image: url( <?php echo $thumb_url_full; ?> );
			}
		}
	</style>