Edit in JSFiddle

<h2>会随着字体变♂大!</h2>
<div class="f20"><img src="//placehold.it/100X100/ccc" alt="">pencil</div>
<div class="f50"><img src="//placehold.it/100X100/ccc" alt="">pencil</div>
<h2>没有字体时侯也会哟!</h2>
<div class="f20"><img src="//placehold.it/100X100/ccc" alt=""></div>
<div class="f50"><img src="//placehold.it/100X100/ccc" alt=""></div>

body {
    font-family:'Helvetica Neue','Microsoft Yahei','Hiragino Sans GB','WenQuanYi Micro Hei',Sans-serif;
    margin: 50px;
    background-color: #eee;
}
div {
    display: inline-block;
    margin-bottom: 5px;
    border: 1px solid #666;
    font-size: 50px;
}
.f20 {
    font-size: 20px;
}
.f50 {
    font-size: 50px;
}