If you use admintools to create your htaccess
SIGpro assumes that the contents of the cache folder must be world-readable, as they directly link to a file inside Joomla!'s cache directory. You can easily work around this, though. Go to the .htaccess Maker page and expand the Server Protection pane. Look in the Fine-Tuning tab and find the "Front-end directories where file type exceptions are allowed" setting. In a new line at the end of the list add
and then click on "Save and create .htaccess". This should restore SIGPro's thumbnails.