How to add a gradient border above an image (diagonal border)

I need something like this

But above image, here is my first try, but it doesn't work.

http://jsfiddle.net/wo8gbhx3/17/

And this is my markup (now)

Html

<div class="testing">
    <ul>
        <li class="selected unavailable">
            <a href="#">
                <img src="http://placehold.it/25x25"/>
            </a>
        </li>
    </ul>
</div>

      

        
        
        
      

    

CSS

.testing ul {
    list-style: none;
}
.testing ul li {
    width: 25px;
    height: 25px;
    position: relative;
}
.testing ul li img {
    width: 100%;
    height: 100%;
}
.unavailable:before {
    position: absolute;
    border: 2px solid green; /* Just for testing */
    background:repeating-linear-gradient( 150deg, #FFF, #FFF 16px, #000 18px);
}