There are some issue with latest flash player that are breaking the fade transition style of the slideshow filemod you are using. I would recommend moving to use normal gallery slideshow instead, with folder parameters to control the style of the slideshow.
Navigate to your "accueil" folder in the admin, and set
use as[folder], and in the
parameters field further down, paste these parameters:
settings.startmode=image&image.slideshow.autostart=true&thumbnails.thumbnailscroller.enabled=false&controls.largecontrols.enabled=false&controls.maincontrols.enabled=false&image.imagebuttons.enabled=false&image.closebutton.enabled=false
It will look similar to what you already got, here is a demo
http://goo.gl/Rp6Jn
Furthermore, if you want to change the interval, you need to add the following to the end of the parameters string above. Change the number to the amount of seconds you want:
&image.slideshow.interval=5
Here is more info about folder parameters if you want to understand how it works:
https://www.photo.gallery/documentation/folder-parameters/