Can I achieve this layout with flexbox with the below document structure?
I want the big <img> on the left with two smaller images on the right and wrapping.
This is what I did, with display: flex on gallery-container and flex-wrap. 
.container {
  height: 100%;
}
.container .gallery-container {
  background-color: #f6f6f6;
  display: flex;
  flex-wrap: wrap;
  width: 300px;
  align-items: flex-start;
}
.container .gallery-container .gallery-big-image {
  display: block;
  width: 200px;
  height: 200px;
  background: lavender;
}
.container .gallery-container .gallery-small-img {
  display: block;
  width: 100px;
  height: 100px;
  background-color: purple;
}<div class="container">
  <div class="gallery-container">
    <div class="gallery-big-image">big</div>
    <div class="gallery-small-img">small</div>
    <div class="gallery-small-img">small</div>
    <div class="gallery-small-img">small</div>
    <div class="gallery-small-img">small</div>
    <div class="gallery-small-img">small</div>
  </div>
</div>(codepen)

 
    