The issue appears to be only present on your homepage with the slideshow working fine in for example here...
http://www.jpleitao.pt/index.php/en/about-us/historial
This would suggest that there is a conflict with one of the modules on your homepage. Try disabling each of your homepage modules one by one until the slideshow functions to find out which module is causing the conflict.
We were unable to try this ourselves as caching is turned on which you will have to disable first.
Ciarán