Add this code to the Custom CSS field on the Settings tab of your Banner widget’s settings:
.global-styles,
[id*="eapps-banner"]
[class*="Background__Base"] {
background-image: url(URL_FOR_DESKTOP) !important;
}
@media(max-width: 550px) {
.global-styles,
[id*="eapps-banner"]
[class*="Background__Base"] {
background-image: url(URL_FOR_MOBILE) !important;
}
}
Do not forget to replace URL_FOR_DESKTOP and URL_FOR_MOBILE with the image links