The HTML code looks like this:

<div id="wrapper">
  <div class="gallery">
    <img src="your image here" class="thumb_1">
    <img src="your image here" class="thumb_1">
    <img src="your image here" class="thumb_1">
    <img src="your image here">
  </div>
  <div class="gallery">
    <img src="your image here" class="thumb_1">
    <img src="your image here" class="thumb_1">
    <img src="your image here" class="thumb_1">
    <img src="your image here">
  </div>
  <div class="gallery_2">
    <img src="your image here" class="thumb_1">
    <img src="your image here" class="thumb_1">
    <img src="your image here" class="thumb_1">
    <img src="your image here">
  </div>
</div>

The stylesheet looks like this:

#wrapper {
    width: 900px;
    padding-top: 55px;
    padding-bottom: 55px;
    margin-right: auto;
    margin-left: auto;
    border: 1px solid #333333;
    background-color: #CCCCCC;
}

.gallery {
    width: 780px;
    margin-right: auto;
    margin-left: auto;
    margin-bottom: 20px;
}

.gallery_2 {
    width: 780px;
    margin-right: auto;
    margin-left: auto;
}

 .thumb_1 {
    margin-right: 20px;
}