<ul id="id1">
<li id="id-facebook">
<a href="https://www.facebook.com/foo">
<img src="https://example.com/images/facebook.svg" width="18" height="18" alt="Facebook">
</a>
</li>
</ul>
#id-facebook a {
position: relative;
z-index: 1;
}
#id-facebook a:after {
content: '';
position: absolute;
top: 0; right: 0; bottom: 0; left: 0;
background-image: url(...);
background-size: cover;
z-index: 2;
}