<form method="get" id="searchform" action="<?php echo home_url( '/' ); ?>">
  <div>
    <input type="text" value="" name="s" id="s" placeholder="Search..." />
    <input type="image" src="<?php echo get_template_directory_uri();?>/images/search.png" id="searchsubmit" value="Search" alt="Search" />
  </div>
</form>