I don't think http and https would have matter, https is like a standard nowadays, in order to check properly, please try the following:
Check your image link and make sure it is working
Visit this FB debugger page - https://developers.facebook.com/tools/debug/ and check if something else is wrong.
According to the documentation, the only required files in a theme are style.css and index.php. All other files are optional. You can use the ones you need and discard the ones you don't need.
If you haven't seen it, I'd recommend you check out WordPress's developer site on the topic of themes.
There are better ways, but one easy way to achieve this if you wanted to customise the search results is to edit the search.php template and make the WP_Query a custom one:
$args = array(
's' => get_search_query(),
'post_type' => array('page'),
$search_query = new WP_Query($args);
if ( have_posts() ) :
while ( $search_query-&...