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..