To achieve the desired effect, the image should be placed inside a circle with a white background.
The image size should be smaller than the circle and centered within it.
One possible way to accomplish this is:
.icon i {
color: #fff;
width: 40px;
height: 40px;
border-radius: 20px;
font-size: 25px;
text-align: center;
margin-right: 10px;
padding-top: 15%;
}
<li><a href="#"><i class="icon" style="background: red;"> <img src="http://www.pngdot.com/wp-content/uploads/2015/11/Free_Arrow_Png_01.png" style="width:50px;height:45px;"></i></a></li>