According to product page source, thumbnail image should be in files/imagecache/product/files/ directory:

<div class="product_image"><a href="http://localhost/ubercart/files/image.jpg" title="" class="thickbox" rel="field_image_cache_0"><img src="http://localhost/ubercart/files/imagecache/product/files/image.jpg" alt="" title=""  /></a><br /></div><div class="display_price">$12.00</div><div class="model">p_description</div><p>p_name</p>

however I do not have that directory at all.
Even creating it manually and adding a new product with an image, the problem still occurs.