#block-menu-menu-social-links .is-leaf:nth-child(3) a {
    background: url(https://www.globaltaxjustice.org/sites/all/themes/gatj/images/instagram.png) no-repeat 0 0;
}

#block-menu-menu-social-links .is-leaf:nth-child(4) a {
    background: url(https://www.globaltaxjustice.org/sites/all/themes/gatj/images/youtube.png) no-repeat 0 0;
}