Spacing in <li> tags with display table-cell

Question

I've four li tags and they all contains images in it. Now, I want to show images vertically center and for this I've used display: table-cell for all li and on image I've used vertical-align middle. But margin isn't working when I am trying to put spacing between li's.

Here is the JSFiddle

HTML

<div class="achievements">
     <h2>Our <span>Achievements</span></h2>

    <ul>
        <li>
            <img src="images/award-1.png">
        </li>
        <li>
            <img src="images/award-2.png">
        </li>
        <li>
            <img src="images/award-3.png">
        </li>
        <li>
            <img src="images/award-4.png">
        </li>
    </ul>
</div>

CSS

.achievements ul {
    margin: 0;
    padding: 0;
    list-style: none;
}
.achievements ul li {
    min-width: 304px;
    min-height: 208px;
    vertical-align: middle;
    display: table-cell;
    box-sizing: border-box;
    -webkit-box-sizing: border-box;
    -moz-box-sizing: border-box;
    box-shadow: 0px 0px 3px #aaa;
    color: #000;
    margin-right: 14px;
    padding-bottom: 32px;
    max-width: 303px;
    background: #fff;
    font-family:'gafata';
    padding-top: 32px;
    line-height: 23px;
    text-align:center;
}
.achievements ul li img {
    vertical-align: middle;
}

Answer 1

Use border-spacing to add spacing between the cells.

In order for this to have an effect on the table, the border-collapse value should be separate (the default value) as opposed to collapse.

Updated Example

.achievements ul {
    margin: 0;
    padding: 0;
    list-style: none;
    display:table;
    border-spacing: 10px;
    border-collapse: separate;
}

If you want to center the table, simply add margin:0 auto. (example)