Optimize search form background image

Update search background image with multiple sizes and added WebP formats.

www/templates/search-form.tpl.php

@@ -2,7 +2,7 @@
   <form action="./" method="get">
       <div class="dcf-input-group">
         <input type="text" name="q" id="search_q" aria-label="Search Query" placeholder="e.g., Herbert Husker, Ph.D." />
-        <span class="wdn-input-group-btn"><button class="button" type="submit"><span class="wdn-icon-search" aria-hidden="true"></span><span class="dcf-sr-only">Search</span></button></span>
+        <button class="button" type="submit"><span class="wdn-icon-search" aria-hidden="true"></span><span class="dcf-sr-only">Search</span></button>
     </div>
       <?php if (!empty($local_results)): ?>
       <input type="hidden" name="u" value="<?php echo htmlentities($_GET['u'], ENT_QUOTES) ?>" />