That happens because you're not escaping the variable correctly.
The following is the right way:
PHP Code:
$template_hook['postbit_userinfo_right_after_posts'] .=
'<br><dt>Nominate for Members Choice Award </dt> <dd><a href="testingthread.php?threadid=' . $threadinfo['title'] . '"> <img src="images/buttons/memchoice.png" style="width: 26px; height: 26px; border: 0px;" alt="Nominate This Poem for Members Choice Award. Thank you."></a></dd>';