Fixes the SERVER variable DOCUMENT_ROOT for VirtualDocumentRoot in the thumbnail generator.

     - If a host uses a VirtualDocumentRoot the SERVER varialbe DOCUMENT_ROOT point to the wrong location. This is a fix for that.
     - Yes, I am aware this was a stupid oversight to fix this in the header file and forget about the thumbnails. One of these days I really shoudl create a common INC file to include in all three (header/footer/thumbnail) files.