If you look at the source of the page and locate the right sidebar, you will see
<!-- Right Sidebar -->
<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
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.