put image in header

hello i want to put a background image in header in full width
i try this in custom.css

.container.t3-header {

  background-image: url("../images/banniere.jpg&quot !important;

}

but it only in width site
how to use full width ?
when i use

.container.t3-header {

  background-image: url("../images/banniere.jpg&quot !important;

  width:100%;

  margin-left: auto;

  margin-right: auto;

}

the header is align to right ... not center
thanks for any help