.square, .circle {
    width: 100px;
    height: 100px;
    display: inline-block; 
    float: left;
}

.circle {
    border-radius: 50%;
}

#blockRed {
    background-color: red;
    position: relative;
}

#blockYellow {
    background-color: yellow;
    position: relative; 
}

#blockSilver {
    background-image: url('https://encrypted-tbn0.gstatic.com/images?q=tbn:ANd9GcQzn01bHqcJOrkCFScTb0fl_cqMD74g_Ge-jGncgoOUIw&s')
}

#container {
    background-color: blue;
    margin: 0 auto;
    width: 400px;
    height: 400px;
}