Ok, after I received your Private message, I did some research. First of all, I can tell you that thumbnails are working here in Firefox:
So why is this? Its something related to filenames or folder names I think -
http://s174403510.onlinehome.fr/Gallery ... 20pile.jpg - Where is this thumbnail? The other thumbnails seem to load fine (in firefox), but you have incredibly long file- and folder names with spaces and special characters. This could create some problems depending on your server. It could be the "." punctuation, or the very long name.
I think its to do with filenames, also because the flaw seems to be browser-related ...