Yep, as Bop says it's probably down to a firewall.
Sounds like you have a router between the internet and your 'in-home network'. What make/model of router is it? We can then advise how to open up the firewall for http (port 80) traffic.
However, I would personally be very wary of opening a home PC to the internet with a web server, esp an old OS like 98 with plenty of holes waiting to be exploited. Win98 is NOT a suitable OS for presenting to the internet. If you really want to have a private webserver, a Linux (or even Solaris) server running Apache would be a better bet, but not trivial to configure.
Why not just upload your photos to your NTL homepage?
to CF btw.