WooCommerce Change AJAX Spinner Gif On Checkout

woocommerce-ajax-checkout-spinner.css

/* 
 * Custom AJAX spinner on WooCommerce checkout 
 * The class used to load the overlay is .blockUI .blockOverlay
 * The class used to load the spinner is .woocommerce .loader:before
 *
 */
.woocommerce .blockUI.blockOverlay:before,.woocommerce .loader:before {
    height: 3em;
    width: 3em;
    position: absolute;
    top: 50%;
    left: 50%;
    margin-left: -.5em;
    margin-top: -.5em;
    display: block;
    content: "";
    -webkit-animation: none;
    -moz-animation: none;
    animation: none;
    background: url('http://test-site.com/wp-content/themes/boss-child/images/spinning-logo.gif') center center;
    background-size: cover;
    line-height: 1;
    text-align: center;
    font-size: 2em;
}

@emmust47 think you just need to add

content: "";