wagtail/client/scss/components/_media-placeholder.scss

.media-placeholder {
    width: 600px;
    height: 400px;
    background-color: #ccc;
    padding: 5px;

    h3,
    p {
        margin: 0;
    }

    img {
        max-width: 350px;
        max-height: 350px;
        margin: 20px;
    }
}