It's in the top-left corner of the Paint window. A menu will appear. This is in the middle of the menu. Doing so opens a File Explorer window. Go to the file location of the photo that you want to open in Paint, then click the photo to select it. It's in the bottom-right corner of the window. Your photo will be uploaded to Paint, and your Paint canvas should resize to fit the photo. If you ever want to open an image in Paint when Paint isn't already open, you can right-click the image, select Open with in the drop-down menu, and click Paint in the resulting pop-out menu.

TL;DR in Chinese:
点击“画图”窗口左上角的文件。 点击菜单中间的打开。 选择一个图片。 点击窗口右下角的打开。 使用右键菜单在“画图”打开图片。