Answered: how to add image banner at the top of the main page only and not over the side bar?

Widget Anywhere might be the plugin you're looking for.
Otherwise If you're looking to display a welcome banner, check out the Welcome Widget.

Edit: DIY manually

If you want the banner to fill both the width of the main content and sidebar, go to:
SnowFlat theme > open file qa-theme.php after line 263 paste the following code:

$this->output('<img style="display:block;margin:0 auto;" src="https://i.ibb.co/dLq5QyJ/z897231.png">');

Should look something like this (screenshot):

public function body_content()
{
    ...
    $this->output('<div class="qa-main-wrapper">', '');
    $this->output('<img style="display:block;margin:0 auto;" src="i.ibb.co/dLq5QyJ/z897231.png">');
    $this->main();
    ...
}

If you want the banner to fill the width of the main content only, and not the sidebar, you'll need to add this function to your qa-theme.php instead. Paste it on line 275 for example (screenshot):

public function main()
{
$content = $this->content;
$hidden = !empty($content'hidden']) ? ' qa-main-hidden' : '';
$this->output('<div class="qa-main' . $hidden . '">');
    $this->output('<img style="display:block;margin:0 auto;" src="i.ibb.co/dLq5QyJ/z897231.png">');   
    $this->widgets('main', 'top');
    $this->page_title_error();
    $this->widgets('main', 'high');
    $this->main_parts($content);
    $this->widgets('main', 'low');
    $this->page_links();
    $this->suggest_next();
    $this->widgets('main', 'bottom');
    $this->output('</div> <!-- END qa-main -->', '');
}

This rewrites the  main()  function of your theme to add the new banner (See highlighted code). 
Although I believe Widget Anywhere should be able to add content there, as I see there's a widgets main top there, on the  main()  function..