import java.awt.BorderLayout;
import java.awt.Image;

import javax.swing.ImageIcon;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JPanel;


public class tt extends JFrame  {
    public  tt() {
        JPanel jPanel=new JPanel();
        ImageIcon image=new ImageIcon("C:/Users/wenguang/Desktop/新建文件夹/IMG_0030.jpg");//添加图片
        
        JLabel label=new JLabel(image);
        int width = image.getIconWidth();//获取图片的宽高
        int height = image.getIconHeight();
        if (width > 1000) {
            width = 1276;
            height = 850;
        }
        image.setImage(image.getImage().getScaledInstance(width, height,//缩小图片
                Image.SCALE_DEFAULT));
        jPanel.add(label,new BorderLayout().CENTER);
        getContentPane().add(jPanel,new BorderLayout().CENTER);
        setBounds(100, 100, 1276, 850);
        setDefaultCloseOperation(EXIT_ON_CLOSE);
        setVisible(true);
    }
    public static void main(String[] args) {
        new tt();
    }

}

import java.awt.BorderLayout;
import java.awt.Image;

import javax.swing.ImageIcon;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JPanel;


public class tt extends JFrame  {
    public  tt() {
        JPanel jPanel=new JPanel();
        ImageIcon image=new ImageIcon("C:/Users/wenguang/Desktop/新建文件9Y/IMG_0030.jpg");
        
        JLabel label=new JLabel(image);
        int width = image.getIconWidth();
        int height = image.getIconHeight();
        if (width > 1000) {
            width = 1276;
            height = 850;
        }
        image.setImage(image.getImage().getScaledInstance(width, height,
                Image.SCALE_DEFAULT));
        jPanel.setLayout(new BorderLayout());jPanel.add(label, BorderLayout.CENTER);//jPanel.add(label,new BorderLayout().CENTER);
        getContentPane().add(jPanel,new BorderLayout().CENTER);
        setBounds(100, 100, 1276, 850);
        setDefaultCloseOperation(EXIT_ON_CLOSE);
        setVisible(true);
    }
    public static void main(String[] args) {
        new tt();
    }

}

关键的修改
修改前：jPanel.add(label,new BorderLayout().CENTER);
修改后：jPanel.setLayout(new BorderLayout());jPanel.add(label, BorderLayout.CENTER);

参考资料：1和2是英文说明，3是官方的示例代码
1）https://docs.
2）https://docs.
3）https://docs.

谢谢，解决啦