I changed from input type = "submit" to input type= "image" .The part of codes between space. i also put the text box and the button into a table. So it can look in a straight line. is this the right way to do it ? not sure , but it works ! 
Code:
*/
$content = " ";
$content .= zen_draw_form('quick_find_header', zen_href_link(FILENAME_ADVANCED_SEARCH_RESULT, '', 'NONSSL', false), 'get');
$content .= zen_draw_hidden_field('main_page',FILENAME_ADVANCED_SEARCH_RESULT);
$content .= zen_draw_hidden_field('search_in_description', '1') . zen_hide_session_id();
if (strtolower(IMAGE_USE_CSS_BUTTONS) == 'yes') {
$content .= '<div id="navMainSearch">' . zen_draw_input_field('keyword', '', 'size="6" maxlength="30" style="width: 150px" value="' . HEADER_SEARCH_DEFAULT_TEXT . '" onfocus="if (this.value == \'' . HEADER_SEARCH_DEFAULT_TEXT . '\') this.value = \'\';" onblur="if (this.value == \'\') this.value = \'' . HEADER_SEARCH_DEFAULT_TEXT . '\';"') . ' ' . zen_image_submit (BUTTON_IMAGE_SEARCH,HEADER_SEARCH_BUTTON) . '</div>';
} else {
// $content .= '<div id="navMainSearch" id="GO">' . zen_draw_input_field('keyword', '', 'size="6" maxlength="30" style="width: 150px" value="' . HEADER_SEARCH_DEFAULT_TEXT . '" onfocus="if (this.value == \'' . HEADER_SEARCH_DEFAULT_TEXT . '\') this.value = \'\';" onblur="if (this.value == \'\') this.value = \'' . HEADER_SEARCH_DEFAULT_TEXT . '\';"') .' '.zen_draw_pull_down_menu('categories_id', zen_get_categories(array(array('id' => '', 'text' => All)), '0','', '1' ), $sData['categories_id']). ' <input type="submit" value="' . HEADER_SEARCH_BUTTON . '" style="width: 45px" /></div>';
$content .= '<div id="navMainSearch">'.
'<table style="text-align: left; width: 410px;" border="0" cellpadding="0" cellspacing="0">
<tbody>
<tr>
'.'<td>'. zen_draw_input_field('keyword', '', 'size="6" maxlength="30" style="width: 150px" value="' . HEADER_SEARCH_DEFAULT_TEXT . '" onfocus="if (this.value == \'' . HEADER_SEARCH_DEFAULT_TEXT . '\') this.value = \'\';" onblur="if (this.value == \'\') this.value = \'' . HEADER_SEARCH_DEFAULT_TEXT . '\';"') .' '.zen_draw_pull_down_menu('categories_id', zen_get_categories(array(array('id' => '', 'text' => All)), '0','', '1' ), $sData['categories_id']).'</td>'.'<td>'.
' <input type="image" src="http://yoursite.com/images/icons/search_button.gif" value="" height="26" type="image" width="35">'
.'</td>'.'</tr></tbody></table>'.'</div>';
}
// $content .= zen_draw_pull_down_menu('categories_id', zen_get_categories(array(array('id' => '', 'text' => All)), '0','', '1' ), $sData['categories_id']);
$content .= "</form>";
?>