SOURCE

<p class="p_0">background-size：默认为 图片原大小</p>
<p class="p_1">background-size：50%</p>
<p class="p_2">background-size：50% 100%</p>
<p class="p_3">background-size：cover 足够大(以宽/高中小的一个做100%)</p>
<p class="p_4">background-size：contain</p>

p{
    margin-top: 10px;
    padding: 10px;
    border: 10px solid rgba(10,10,10,.2);
    background-color: yellowgreen;
    background-image:url(https://www.w3school.com.cn/i/bg_flower.gif);
    background-repeat: no-repeat;
}
.p_1{
    background-size: 50%;
}
.p_2{
    background-size: 50% 100%;
}
.p_3{
    background-size: cover;
}
.p_4{
    background-size: contain;
}