identify which of my plugins php files 'view source' elements come from to edit

I’m using a jquery plugin to create an image slider with thumbnails underneath. My goal is to have the main image’s background be bg.png, with the thumbnail section background be wood.png.

I’ve found the following using ‘view source’ in FF:

<div class="thethe_image_slider white-square-1" id="thethe_image_slider323" style="width:921px;"><div class="thethe_image_slider_inner" style="width:921px; height:392px;">

which I would like to change to this:

<div class="thethe_image_slider white-square-1" id="thethe_image_slider323" style="width:961px;"><div class="thethe_image_slider_inner" style="width:921px; height:392px;margin:0 auto;">

I’ve searched through all of my plugin’s php files and have found nothing similar to what view source as generated- how do I access that info OR make the changes?

live site.

Thanks.