Well, for a link, there must be a link tag around. what you can also do is that make a css class for the button and assign that class to the link tag. like,
#btn {
background: url(https://image.flaticon.com/icons/png/128/149/149668.png) no-repeat 0 0;
display: block;
width: 128px;
height: 128px;
border: none;
outline: none;
}