I'm using Joomla 1.0.12 and yt_beyond template from YOOtheme.
The plugin works fine without any problems but suddenly with the new articles blank thumbnails appears without images. I read the same problem in the forum and checked the following:

1. Plugin folder permission 755
2. Clean all caches
3. Cache folder permission 755
4. All images files are jpg
5. GD library installed

This is the website:
www.kindergartensudoku.com
from the left menu (Children Story Books)> My Father's Dragon or Treasure Island.
In other articles the thumbnails appear.
:-\

SOLVED SOLVED. The size of the images was to big more than one megabyte for each image, I re sized all the images less than 100 kilobyte and the width less than 500 pixles for every image and its works.  :)