[Mooff]

The slider does not care what size your images are.
Your articles either need to have a preview image - or you have to set "show_articles_without_image" to true in the options, then it will show a default image you can specify for articles without image.

What image size do you want the slider to use? That's a question only you can answer.

Template search? You mean for the other jquery files? You can see what they are called if you check the page via source code (in firefox ctrl+u).
You have this one:
<script type="text/javascript" src="/forum/arrowchat/includes/js/jquery.js"></script>
and that one:
<script type="text/javascript" src="steamprofile/jquery-1.3.2.min.js"></script>
and of course the slider one:
<script type="text/javascript" src="clientscript/jquery/jquery-1.6.1.min.js"></script>

As for guessing, the other ones come from a chat addon and a steam addon.