If I have HTML like this:
<a class="facebook" href="https://www.facebook.com/">Facebook</a>
I can use this CSS
a#facebook{
background:#666;/*add facebook image background here*/
text-indent:-999999px;
display:block;
overflow: hidden;
width:100px;/*match with your facebook background's width*/
height:100px;/*match with your facebook background's height*/
}
Note that, It is not a good practice because if user disable image, but not css, it will be no text to tell them what is in that space