You're missing the dot before the class. Also, what are you trying to aim with the "10" and "15"? If you're trying to set width and height, state those inside of the class instead, just like you did.
It should be:
.li_img {
width:28px;
height:32px;
background:url(https://ssl.gstatic.com/gb/images/v1_90902864.png);
}