Ok finaly i find a solution, i use one image in css for landing page and a other one in png for connected people.
Thank's @ketkew who put me of the way of  CSS Background Patterns 

body.default-body
{
padding-top: 125px;
background-color: #ffffff;
background-image: url("https://www.transparenttextures.com/patterns/purty-wood.png");
}


/* page d'accueil invite */
#page_guest_users-member_login #content-wrapper.container, #page_guest_home-index .container#content-wrapper {
width: 100%; !important;
padding: 0; !important;
margin: 0; !important;
/* fond carreau rouge page d'accueil invite */
background-image: linear-gradient(90deg, rgba(200, 0, 0, .5) 50%, transparent 0),
linear-gradient(rgba(200, 0, 0, .5) 50%, transparent 0);
background-size: 20px 20px;
}
/* end */

Ok finaly i find a solution, i use one image in css for landing page and a other one in png for connected people.
Thank's @ketkew who put me of the way of  CSS Background Patterns 

body.default-body
{
padding-top: 125px;
background-color: #ffffff;
background-image: url("https://www.transparenttextures.com/patterns/purty-wood.png");
}


/* page d'accueil invite  - Landing page*/
#page_guest_users-member_login #content-wrapper.container, #page_guest_home-index .container#content-wrapper {
width: 100%; !important;
padding: 0; !important;
margin: 0; !important;
/* fond carreau rouge page d'accueil invite */
background-image: linear-gradient(90deg, rgba(200, 0, 0, .5) 50%, transparent 0),
linear-gradient(rgba(200, 0, 0, .5) 50%, transparent 0);
background-size: 20px 20px;
}
/* end */