Hello, Phil!
Here is the working solution for you:
Open the Customizing->General->Additional CSS
and Copy/Paste the code below :
@media only screen and (max-width: 479px) {
.pull-left a img{
display: none;}
.pull-left a {
background-image: url(https://banner2.kisspng.com/20180325/txe/kisspng-knight-cartoon-clip-art-knights-
cliparts-public-domain-5ab74f63119b04.7085046415219628510721.jpg);
background-size: contain!important;
width: 150px;
height: 150px;
background-position: center;
background-repeat: no-repeat;
margin-left:auto;
margin-right:auto;
}
}
Replace the image URL and change the width and height values per your needs.
Regards