<div class="demo">
        <div class="grid">
          
            <div class="wrapper">
                <img class="img" src="http://www.lesoir.be/sites/default/files/dpistyles_v2/ena_16_9_extra_big/2018/07/30/node_170571/23034343/public/2018/07/30/B9716482249Z.1_20180730173955_000+GBUBP8UKN.1-0.jpg?itok=28M5ZZkG">
            </div>
          
            <div class="wrapper">
                <img class="img" src="http://media.comicbook.com/2018/03/stranger-things-cast-salary-season-3-netflix-1094826-1280x0.jpeg">
            </div>
          
            <div class="wrapper">
                <img class="img" src="https://pmctvline2.files.wordpress.com/2018/03/st.jpg?w=620&h=420&crop=1">
            </div>
          
            <div class="wrapper">
                <img class="img" src="https://otakukart.com/wp-content/uploads/2018/07/Stranger-Things-Season-3.jpg">
            </div>
          
        </div>
    </div>

.img {
  width: 100%;
  height: auto;
  margin: 2px;
  max-width: 400px;
}

.grid {
  display: flex;
  flex-wrap: wrap;
}