As far as I know - you can only use .gif .jpg and .png
I'm pretty sure that .jpeg won't appear in all browsers - it's mislabled and should have the three letter extension. .jpg
So modify knowing that this may be bad.

But this line:
$Body = preg_replace("/(\[IMG\]|\[{$ubbt_lang['TEXT_IMAGE']}\])http([^\[]*)\.(gif|jpg|png)(\[\/IMG\]|\[\/{$ubbt_lang['TEXT_IMAGE']}\])/i","<img src="http\\2.\\3">",$Body);
in ubbt.inc.php
You could change this bit:
(gif|jpg|png)
to this:
(gif|jpg|png|jpeg)