width: 100%;height: 100%;object-fit:fill;

.wrapper_image{
    max-width:100%;
    max-height:100%;
    min-height:100%;
    min-width: 100%;
    margin: 0 auto;
}

<div class="o-grid__col u-12/12" style="width: 400px;
          height: 400px;
          position: relative;
          display: inline-block;
          overflow: hidden;
          text-align: center;
          margin: 0;">
          <div class="mySlides wrapper_image_horses">
          <img src='url' style="width: 100%;height: 100%;object-fit:fill;">
          </div>
</div>