Hi Basilis,

Thank your for your answer, here is my code I used to fix my problem:

.avia-slideshow ul.avia-slideshow-inner {
height: 400px !important;
}

.avia-slideshow li img {
 position: absolute;  
    top: 400px;  
    bottom: 0;  
    left: 0;  
    right: 0;  
    margin: auto;
}