Babylon.js/Viewer/assets/templates/default/loadingScreen.html

<style>
    /* Loading Screen element */

    loading-screen {
        position: absolute;
        z-index: 100;
        opacity: 1;
        pointer-events: none;
        display: flex;
        justify-content: center;
        align-items: center;
        -webkit-transition: opacity 2s ease;
        -moz-transition: opacity 2s ease;
        transition: opacity 2s ease;
    }

    img.loading-image {
        -webkit-animation: spin 2s linear infinite;
        animation: spin 2s linear infinite;
    }

    @-webkit-keyframes spin {
        0% {
            -webkit-transform: rotate(0deg);
        }
        100% {
            -webkit-transform: rotate(360deg);
        }
    }

    @keyframes spin {
        0% {
            transform: rotate(0deg);
        }
        100% {
            transform: rotate(360deg);
        }
    }
</style>

<img class="loading-image" src="{{loadingImage}}">