.cb-slideshow li:nth-child(1) span { 
    background-image: url(../img/perso1.jpg); 
}

.cb-slideshow li:nth-child(2) span { 
    background-image: url(../img/perso2.jpg); 
}

.cb-slideshow li:nth-child(3) span { 
    background-image: url(../img/perso3.jpg); 
}

.cb-slideshow li:nth-child(4) span { 
    background-image: url(../img/perso4.jpg); 
}

.cb-slideshow li:nth-child(5) span { 
    background-image: url(../img/perso5.jpg); 
}

.cb-slideshow li:nth-child(6) span { 
    background-image: url(../img/perso6.jpg); 
}