If you want to use a Discourse emoji and have it at the same place as the original emoji, you can use the following code:
.waving-hand {
display: none;
}
.login-title:after {
content: "";
background: url("https://emoji.discourse-cdn.com/twitter/slight_smile.png?v=12");
background-size: contain;
display: inline-block;
height: 35px;
width: 35px;
margin-left: 35px;
}