.imagen { 
    width: 275px;
    height: 338px;
    margin: 0 5px;

}
.contenedor{
    display: flex;
    justify-content: center;
}

@media (max-width: 900px){

    .contenedor{
        flex-direction: column;
        align-items: center;
    }
    .imagen{
        width: 418px;
        margin-bottom:  5px;
    }
}