但是,由于某些原因,它们的行为不正确,因为它们的大小不同.

他们必须表现得像这样：

/* ================================= 
      Base Styles
    ==================================== */

body {
  display: flex;
  flex-direction: column;
  min-height: 100vh;
}
.row {
  flex: 1;
}
/* ================================= 
      Media Queries
    ==================================== */

@media (min-width: 769px) {
  .main-header,.main-nav,.row {
    display: flex;
  }
  .main-header {
    flex-direction: column;
    align-items: center;
  }
  .col {
    flex: 1 50%;
  }
  .row {
    flex-wrap: wrap;
  }
  .item-2 {
    order: -1;
  }
}
@media (min-width: 1025px) {
  .main-header {
    flex-direction: row;
    justify-content: space-between;
  }
  .col {
    flex-basis: 0;
  }
  .item-1 {
    flex-grow: 1.4;
  }
}

<div class="row">

  <div class="item-1 col">
    <img src="http://blackrockdigital.github.io/startbootstrap-freelancer/img/portfolio/cabin.png">
  </div><!--/.primary-->

  <div class="item-2 col">
    <img src="http://blackrockdigital.github.io/startbootstrap-freelancer/img/portfolio/cabin.png">
  </div><!--/.secondary-->

  <div class="item-3 col">
    <img src="http://blackrockdigital.github.io/startbootstrap-freelancer/img/portfolio/cabin.png">
  </div><!--/.tertiary-->

  <div class="item-4 col">
    <img src="http://blackrockdigital.github.io/startbootstrap-freelancer/img/portfolio/cabin.png">
  </div><!--/.tertiary-->

  <div class="item-5 col">
    <img src="http://blackrockdigital.github.io/startbootstrap-freelancer/img/portfolio/cabin.png">
  </div><!--/.tertiary-->

  <div class="item-6 col">
    <img src="http://blackrockdigital.github.io/startbootstrap-freelancer/img/portfolio/cabin.png">
  </div><!--/.tertiary-->

</div>

你可以在这里查看JSFIDDLE：https://jsfiddle.net/ju157mnj/2/