Could you try the following code change? In /components/com_jfbconnect/libraries/provider/twitter/channel/stream.php around line 78, add
$post->thumbPicture = (isset($tweet->entities->media[0]) ? $tweet->entities->media[0]->media_url : "");
I don't see that code for Twitter images was ever checked in, so if that works for you (it's working on my test site), I'll make sure to get it in our next release.
-Melissa