Estou com dificuldade para usar o background-image na tag <ul class="icones-sociais">

    <div>
        <aside class="barra-navegacao">
        <h1>João da Silva</h1>
            <nav>    
                <ul>
                    <li><a href="index.html">Home</a></li>
                    <li><a href="portfolio.html">Portfolio</a></li>
                    <li><a href="bio.html">Sobre mim</a></li>
                    <li><a href="blog.html">Blog</a></li>
                    <li><a href="contato.html">Contato</a></li>
                </ul>
            </nav>
            <ul class="icones-sociais">
                <li><a href="https://github.com/joaodasilva">Github</a></li>
                <li><a href="https://twitter.com/joaodasilva">Twitter</a></li>
                <li><a href="https://br.linkedin.com/pub/linkedin-joao-da-silva/32/4/508">Linkedin</a></li>
            </ul>
        </aside>
    </div>    
        <footer>
            &copy; João da Silva 2014
            <ul class="icones-sociais social-rodape">
                <li><a href="https://github.com/joaodasilva"><img src="/home/limbin/Downloads/projeto/github.png" alt="github"></a></li>
                <li><a href="https://twitter.com/joaodasilva"><img src="/home/limbin/Downloads/projeto/twitter.png" alt="twitter"></a></li>
                <li><a href="https://br.linkedin.com/pub/linkedin-joao-da-silva/32/4/508"><img src="/home/limbin/Downloads/projeto/linkedin.png" alt="linkedin"></a></li>
            </ul>
        </footer>    
    </body>
</html>

footer {
    background-color: #000;
    color: #F2FFFC;
    padding: 20px;
}
.icones-sociais li{
    display: inline-block;
}
.icones-sociais social-rodape{
    margin: 60px;
}
.icones-sociais{
    background-image: url(github.png);
}