Topic: working for me, not for others...

I run three galleries on my website (http://focale-fixe.net). It works perfectly on my computer, I tested it with firefox and internet explorer.
Some of my friends (not all of them) tells me that they can't get things to work correctly.
They apparently get stuck to the first image and find no mean to see the others whereas they see the thubnails.
I am no computer scientist and don't understand why it may work for some and not for others...
Can anyone help me out ?

Thank you by advance...