* {
  box-sizing: border-box;
}

div {
  margin: 0px;
  border: 0px;
  padding: 0px;
}

.block img {
  width: 100%;
}

.block {
  width: 100%; 
  float: left;
}

@media only screen and (min-width: 512px) {
  .block {
    width: 50%;
  }
}

@media only screen and (min-width: 768px) {
  .block {
    width: 25%;
  }
}