.overlay-container {
    position: relative;
    display: inline-block;
}

.base-image {
    display: block;
    max-width: 90%;
    max-height: 90%;
    border: 4px solid black;
    border-radius: 4px;
}

.overlay-image {
    position: absolute;
    top: 0;
    left: 0;
    width: 100%;     /* same size as base image */
    height: 100%;
    pointer-events: none;
}