.et_pb_posts .et_pb_post a.entry-featured-image-url img,
.archive .et_pb_post a.entry-featured-image-url img,
.search .et_pb_post a.entry-featured-image-url img {
  float: left;
  max-width: 210px !important;
  padding-right: 20px;
}

@media only screen and (max-width: 980px) {
  .et_pb_posts .et_pb_post a.entry-featured-image-url img,
  .archive .et_pb_post a.entry-featured-image-url img,
  .search .et_pb_post a.entry-featured-image-url img {
    float: none;
    max-width: 100% !important; /* could be 400px, default value for et-pb-post-main-image thumbnail */
  }
}