Twenty-Eleven Theme and Automatic Picture Resizing

Here is a quick function to remove the width and height attributes from images which should resolve your issue.

function _remove_img_dimensions_WPA84292($html) {
  $html = preg_replace('/(width|height)=["\']\d*["\']\s?/', "", $html);
    return $html;
}
//for new images
add_filter('post_thumbnail_html', '_remove_img_dimensions_WPA84292', 10);
add_filter( 'image_send_to_editor', '_remove_img_dimensions_WPA84292', 10 );
//for images already inserted into posts
add_filter('the_content', '_remove_img_dimensions_WPA84292', 10);