How to change Form Block lightbox button to Image

#1. First, use below tool to find Form Block ID.

In my example, it is:

• #block-yui_3_17_2_1_1722911720670_3901

#2. Next, use this code to Custom CSS box to replace the button with an image

#block-yui_3_17_2_1_1722911720670_3901 button.lightbox-handle {
    background-image: url(https://cdn.pixabay.com/photo/2018/07/10/21/53/tournament-3529744_1280.jpg) !important;
    color: transparent !important;
    font-size: 0 !important;
    background-size: cover !important;
    background-repeat: no-repeat !important;
    background-position: center center;
    padding: 100px 200px !important;
    border-radius: 0 !important;
}

#3. Result