Thanks for the responses!
I've been working with this code, which has made the main logo disappear on mobile but isn't showing the alt img, so I'm trying to troubleshoot that now, but I'm not getting very far...
/* assuming mobile-first approach */
.navbar .navbar-brand a {
display: block;
background: url(/wp-content/uploads/2020/08/[email protected]) no-repeat;
width: 288px;
height: 64px;
}
.navbar .navbar-brand img {display: none;}
@media screen and (min-width: 37.5em) { /* adapt width to your needs */
.navbar .navbar-brand a {
background: none;
width: auto;
height: auto;
}
.navbar .navbar-brand img {display: block;}
}