wagtail/client/scss/objects/_avatar.scss

// user avatars
.avatar {
    border-radius: 100%;
    position: relative;
    display: inline-block;
    vertical-align: middle;
    text-align: center;
    overflow: hidden;
    width: 50px;
    height: 50px;

    img {
        position: absolute;
        z-index: 2;
        top: 0;
        left: 0;
        right: 0;
        border: 0;
    }

    &.small {
        vertical-align: middle;
        margin: 0 0.5em;
        width: 25px;
        height: 25px;
    }

    &.square {
        border-radius: 0;

        &:before {
            border-radius: 0;
        }
    }
}