#mainimg > img {
    width: 300px;
    height: 300px;

    border: 1px solid purple;
    border-radius: 10px;
    margin: 20px;
}
#thumimg > img {
    width: 100px;
    height: 100px;

    margin: 10px;
    margin-top: 20px;
    box-sizing: border-box;
}

#thumimg > img:hover {
    cursor: pointer;
    border: 4px solid green;
}