Hi ! biffbaxter, Thanks for your answer.
OK I solved the problem of screens settings, I just had to do it twice, one time to set the desktop screen and another time to save the setting.
But I still have the background image not correctly set, the image is not extended to fit the full screen, no big deal I 'll do it by hand every time, I can not save it and so return to fit the laptop screen setting at each reboot.
R8 lxde
Is a laptop hp g7000 a bit old but run good, 2Go Ram and a desktop screen 23' connected on vga, iI like to switch off the laptop screen, but that is ok now