If you look at the source of the page and locate the right sidebar, you will see
HTML Code:
<!-- Right Sidebar -->
<td id="right">
<div id="ngg-images-3" class="widget ngg_images">
<div class="hslice" id="ngg-webslice" >
<h2 class="widgettitle entry-title"></h2>
<div class="ngg-widget entry-content">
<a href="http://www.wheatonriver.net/wp-content/gallery/gallery-home/img_5417.jpg" title="" class="shutterset_sidebar_3"><img src="http://www.wheatonriver.net/wp-content/gallery/gallery-home/thumbs/thumbs_img_5417.jpg" width="100" height="67" title="img_5417" alt="img_5417" /></a>
So this is created by NextGen. By looking at the beginning of the source you can locate the line
HTML Code:
http://www.wheatonriver.net/wp-content/plugins/nextgen-gallery/css/nggallery.css
the css NextGen uses. Now by entering that in the browser address you can look at the CSS and check for each of the ID+ and CLASS= entries to see if any have a css selector associated with it.
When you find one, examine the css rules that are applied. One probably is causing the left justification. When you find it, you can code a CSS Insert to override it.