In the last couple hours I have read more about GD and graphics than I ever wanted to know
Hopefully this will fix it.
This fix only updates the viPortfolio.php file. I am attaching a zip file with just that php file in it. If you could extract this one over the previous one and verify that you now have version 1.0.5.
There are some errors in this version. I haven't not pinpointed the exact cause yet. I get error messages saying: Cannot allocate palotte or Cannot allocate colors. Then a try a different image of the same type and it works fine.
I still have not found out why people are not see the thumbnail images instead seeing a broken link. I am still hopeing the thumbnail creation and the broken links are related.
For those a little hesitant that I even have it running on my server

My test server is
Portfolio Test Server This url will not be valid for very long. I randomly get a new ip address and even more frequently install and uninstall the portfolio. I will leave this available for the next few days though.