Can you please elaborate, please provide examples of the html and what you expect to get as well as what you are getting instead.

From the html:
<html>
<head>
</head>
<body>

</body>
</html>

I got the onclick already:

php> $html = new simple_html_dom();
php> $html->load_file('t.html');
php> $res = $html->find('html/body/img');
php> echo $res[0]->onclick
JavaScript:alert('You clicked?');

If it is not written in the html as per my example above there is no way for the parser to know about it as it is incapable of executing JavaScript in any way, I am afraid.