Pete I did a Whois search for your domain and see that it is hosted on an Apache web server.

Apache is most likely running on a Unix or Linux server which is a case sensitive operating system.

Windows is not case sensitive so Payment_Types.gif and payment_types.gif would be the same on Windows.

On Unix and Linux Payment_Types.gif and payment_types.gif would be two different files.

Not much help but maybe a little background to help isolate the problem.