Hi,

Please try add more this code to Customize > Custom CSS to do that:

.footer-subscribe{
    background-image: url('YOUR_IMAGE_URL_PUT_HERE');
    background-repeat: no-repeat;
    background-position: center center;
    background-size: cover;
    padding: 100px 0 90px;
}
@media only screen and (max-width: 479px){
.footer-subscribe{ padding: 50px 0 40px; }
}

Note Important:

1/ You need to change YOUR_IMAGE_URL_PUT_HERE on the above code to your background image URL.

You can check this guide to know how to get URL of images upload WordPress.

2/ You can change values 100 and 90 on the above code to increase the dimensions of the signup form for desktop and change values 50 and 40 to change the dimensions of the signup form for mobile.

Best Regards,

PenciDesign