You may try by overriding a_list() method into your theme file. Just make sure yoru theme not already have this method else you will get an error
Add below code into your theme file.
function a_list($a_list)
{
//put your code here
//if you are inserting html than use output()
$this->output('your-html-code-here');
qa_html_theme_base::a_list($a_list);
}
Edit: Added code and output image
Code:
function a_list($a_list) {
$this->output('<img src="http://www.q2amarket.com/cdn/adverts/q2am-featured-questions-leaderboard.jpg" style="margin:20px 0; max-width:100%" />');
parent::a_list($a_list);
}