wp-content/themes/twentyten/functions.phpを編集
/*
* Set the content width based on the theme’s design and stylesheet.
*
* Used to set the width of images and content. Should be equal to the width the theme
* is designed for, generally via the style.css stylesheet.
*/
if ( ! isset( $content_width ) )
$content_width = 860; /* 内容部分の横幅の設定 *//* Tell WordPress to run twentyten_setup() when the ‘after_setup_theme’ hook is run. */
add_action( ‘after_setup_theme’, ‘twentyten_setup’ );
これでギャラリーの横幅が大きくなる。