How to use a different sub domain for uploaded files

You’d have to manually edit it for every site.

Go in to Network Admin -> Sites and click on Edit. Go in to options and edit it there.

You should be able to define a constant for this in wp-config.

right Andrea_r
sorry for my lack of connaissance, but how do I do that, or what should I google for to learn?

thanks

I was poking around and the define in wp-config move your whole wp-conent folder, so plugns & themes would be offloaded as well.

Go with what ipstenu suggested.