I re-uploaded the subfolders, so that now seems to work. Now to figure if something's wrong with the authorize.net configuration as I'm getting the following error message now:
The following errors have occurred.
(103) This transaction cannot be accepted.
I'm not sure if my replacing thumbnail with the product ID has anything to do with it though.
Update, I'll post this dilemna as a new question. Seems like one thing after another...