Just to keep track of this on my blog: a great tutorial to install a new image on my OnePlus One and root it...
Click to Read More
The tutorial : http://www.dexteroid.com/how-to/root-install-cyanogen-os-13-oneplus-one/
And the page with all Cyanogen images: https://forums.oneplus.net/threads/mirrors-for-official-cyanogen-roms-ota-updates.141825/
Worked fine ;)