Chnage image for ionic icons

How can I change the image for any icon such as ion-navicon.
I have created a class. i.e.
width: 40px;
height: 40px;
position: relative;
display: inline-block;
margin: 0 0 0 10px;
background-image: url(’…/images/menu_icon.png’);


However it is not showing any image for the button.
Please help me fixing this