nice site...
not sure if you can password protect individual files, maybe you can with a script or something but not sure about that..
but i do know you can use cPanel (Password Protect Directories) to password protect directories. this way you can put the slideshows in different directories which have their own passwords...
www.dublee.com/mnrspmc
you can see it at work there. it's a password protected directory
did you really think i was going to give you the password?!