[Bug Report] 官网图片显示尺寸错误

### Element UI version
2.15.14

### OS/Browsers version
chrome/Edge

### Vue version
2.7.16-beta.1

### Reproduction Link
https://github.com/ElemeFE/element

### Steps to reproduce
Element官网右下tooltips 微信图标，鼠标放上去展示的二维码尺寸错误
对应错误css
.el-popover.footer-popover img {
size: 100px; // size属性不生效
margin: 10px;
}

### What is Expected?
修复官网图片显示尺寸错误

### What is actually happening?
修复官网图片显示尺寸错误