.photoslider{
  max-width: 60%;
  margin-top: 100px;
  margin-bottom: 100px;
  margin-left: auto;
  margin-right: auto;  
}
.mySlides{
  margin-left: auto;
  margin-right: auto; 
  border-radius: 20px; 
}
@media (max-width: 800px) {
  .photoslider{
    margin-top: 50px;
    margin-bottom: -50px;
    max-width: 100%;
  }
}