Retrieve the parameters when searchform action is post

Hi,
I do have a search form.

<form method="get" id="searchform" action="<?php bloginfo('siteurl');?>">
      <div class="search_bx1"><input type="text" class="field" name="s" id="s" placeholder="<?php esc_attr_e( 'Search', 'mclaren' ); ?>" /></div>
      <div class="search_btn1">
      <input type="image" class="submit" name="submit" id="searchsubmit" src="<?php bloginfo('template_directory'); ?>/images/search_btn.jpg" width="22" height="20" value="<?php esc_attr_e( 'Search', 'twentyeleven' ); ?>" />
     </div>
</form>

I need to hide the image co-ordinates from the url.So i think i need to change the form action to post.But when i change the form action the redirection doesn’t occur to search.php rather it goes to index.php.How can i overcome this.Please help?

I follow the default search method followed by the theme ‘twentyeleven’