WordPress adds fixed widths to divs with image captions. Add this code to your functions.php file to remove the fixed width and height so you can properly style with CSS.

wordpress-remove-fixed-width-captions.php

<?php

//------------------------------------------------------------------------------
// Remove height and width automatic from image captions

add_shortcode('wp_caption', 'fixed_img_caption_shortcode');
add_shortcode('caption', 'fixed_img_caption_shortcode');
function fixed_img_caption_shortcode($attr, $content = null) {
	if ( ! isset( $attr['caption'] ) ) {
		if ( preg_match( '#((?:<a [^>]+>\s*)?<img [^>]+>(?:\s*</a>)?)(.*)#is', $content, $matches ) ) {
			$content = $matches[1];
			$attr['caption'] = trim( $matches[2] );
		}
	}
	$output = apply_filters('img_caption_shortcode', '', $attr, $content);
	if ( $output != '' )
		return $output;
	extract(shortcode_atts(array(
		'id'	=> '',
		'align'	=> 'alignnone',
		'width'	=> '',
		'caption' => ''
	), $attr));
	if ( 1 > (int) $width || empty($caption) )
		return $content;
	if ( $id ) $id = 'id="' . esc_attr($id) . '" ';
	return '<div ' . $id . 'class="wp-caption ' . esc_attr($align) . '">'
	. do_shortcode( $content ) . '<p class="wp-caption-text">' . $caption . '</p></div>';
}

?>

Used on my hiking blog to allow my images and captions with hiking directions to be responsive.