dot in image css

#cont {
        width: 200px;
        height: 200px;
        border: 1px solid #aaa; /*To show the boundaries of the element*/
    }
    #cont:before {
        position: absolute;
        content: '';
        background-color:#FF0000;
        border-radius:50%;
        opacity:0.5;
        width: 20px;
        height: 20px;
        pointer-events: none;
    }