When writing a small program today, the background picture cannot be displayed on the mobile phone.
Checked the Q&A of the WeChat applet, the document is written like this
本地资源无法通过 WXSS 获取
background-image：可以使用网络图片，或者 base64，或者使用<image/>标签。
As a novice, this is the first time I saw the base64 statement. Baidu has a base64 decoding/encoder to decode the picture. After trying it, it works.