Remove extra 10px that WordPress adds to captions props to Justin Tadlock: http://justintadlock.com/archives/2011/07/01/captions-in-wordpress

gistfile1.txt

<?php
/**
 * This function removes the extra 10px width that WordPress adds to captions
 *
 * @param $output
 * @param $attr
 * @param $content
 *
 * @return string
 */

function cleaner_caption( $output, $attr, $content ) {

	/* We're not worried abut captions in feeds, so just return the output here. */
	if ( is_feed() )
		return $output;

	/* Set up the default arguments. */
	$defaults = array(
		'id' => '',
		'align' => 'alignnone',
		'width' => '',
		'caption' => ''
	);

	/* Merge the defaults with user input. */
	$attr = shortcode_atts( $defaults, $attr );

	/* If the width is less than 1 or there is no caption, return the content wrapped between the [caption]< tags. */
	if ( 1 > $attr['width'] || empty( $attr['caption'] ) )
		return $content;

	/* Set up the attributes for the caption <div>. */
	$attributes = ( !empty( $attr['id'] ) ? ' id="' . esc_attr( $attr['id'] ) . '"' : '' );
	$attributes .= ' class="wp-caption ' . esc_attr( $attr['align'] ) . '"';
	$attributes .= ' style="width: ' . esc_attr( $attr['width'] ) . 'px"';

	/* Open the caption <div>. */
	$output = '<div' . $attributes .'>';

	/* Allow shortcodes for the content the caption was created for. */
	$output .= do_shortcode( $content );

	/* Append the caption text. */
	$output .= '<p class="wp-caption-text">' . $attr['caption'] . '</p>';

	/* Close the caption </div>. */
	$output .= '</div>';

	/* Return the formatted, clean caption. */
	return $output;
}
add_filter( 'img_caption_shortcode', 'cleaner_caption', 10, 3 );