The property float:centre is not a valid CSS property. To center the image and label containing the name, you need to encapsulate them in their own div or container and give that container a text-align: center;
.img-container{
display: inline-block;
text-align:center;
}
img{
border-radius:100%;
width:200px;
float:left;
margin: 0 20px;
}
After compiling, your HTML would look like this:
<h1>CONTRIBUTORS</h1>
<div id="contributors_image">
<div class="img-container">
<img src="http://fineartamerica.com/images/artworkgallerylogos/yosemite-national-park-science-source-images-1404150568.jpg" />
<h4>John Doe</h4>
</div>
<div class="img-container">
<img src="http://fineartamerica.com/images/artworkgallerylogos/yosemite-national-park-science-source-images-1404150568.jpg" />
<h4>John Doe</h4>
</div>
<div class="img-container">
<img src="http://fineartamerica.com/images/artworkgallerylogos/yosemite-national-park-science-source-images-1404150568.jpg" />
<h4>John Doe</h4>
</div>
<div class="img-container">
<img src="http://fineartamerica.com/images/artworkgallerylogos/yosemite-national-park-science-source-images-1404150568.jpg" />
<h4>John Doe</h4>
</div>
<div class="img-container">
<img src="http://fineartamerica.com/images/artworkgallerylogos/yosemite-national-park-science-source-images-1404150568.jpg" />
<h4>John Doe</h4>
</div>
<div class="img-container">
<img src="http://fineartamerica.com/images/artworkgallerylogos/yosemite-national-park-science-source-images-1404150568.jpg" />
<h4>John Doe</h4>
</div>
</div>
For guidance, refer to the demo, but make sure to modify it according to your specific markup for compilation from your code.