The gdthumb.php was a php file I was using to testing the thumbnail functionality. I have modified it a little bit more to make it a useful testing tool for you also.
I have attached the gdthumb.php file below. If you could copy this to you viPortfolio/ directory and run it with
http://{yoursite}/viPortfolio/gdthumb.php it will try and convert the installerlogo.jpg file to a thumbnail and save it to a file with a link to be able to view the new file.
This will verify if it is my code or something to do with GD or PHP.