参考代码：

from PyQt5.QtWidgets import QWidget, QApplication, QLabel
from PyQt5.QtCore import QRect, Qt
from PyQt5.QtGui import QImage, QPixmap, QPainter, QPen, QGuiApplication
import cv2
import sys
class MyLabel(QLabel):
 x0 = 0
 y0 = 0
 x1 = 0
 y1 = 0
 flag = False
 #鼠标点击事件
 def mousePressEvent(self,event):
  self.flag = True
  self.x0 = event.x()
  self.y0 = event.y()
 #鼠标释放事件
 def mouseReleaseEvent(self,event):
  self.flag = False
 #鼠标移动事件
 def mouseMoveEvent(self,event):
  if self.flag:
   self.x1 = event.x()
   self.y1 = event.y()
   self.update()
 #绘制事件
 def paintEvent(self, event):
  super().paintEvent(event)
  rect =QRect(self.x0, self.y0, abs(self.x1-self.x0), abs(self.y1-self.y0))
  painter = QPainter(self)
  painter.setPen(QPen(Qt.red,2,Qt.SolidLine))
  painter.drawRect(rect)
class Example(QWidget):
 def __init__(self):
  super().__init__()
  self.initUI()
 def initUI(self):
  self.resize(675, 300)
  self.setWindowTitle('在label中绘制矩形')
  self.lb = MyLabel(self) #重定义的label
  self.lb.setGeometry(QRect(30, 30, 511, 541))
  img = cv2.imread('2.jpg')
  height, width, bytesPerComponent = img.shape
  bytesPerLine = 3 * width
  cv2.cvtColor(img, cv2.COLOR_BGR2RGB, img)
  QImg = QImage(img.data, width, height, bytesPerLine,QImage.Format_RGB888)
  pixmap = QPixmap.fromImage(QImg)
  self.lb.setPixmap(pixmap)
  self.lb.setCursor(Qt.CrossCursor)
  self.show()
if __name__ == '__main__':
 app = QApplication(sys.argv)
 x = Example()
 sys.exit(app.exec_())

记得将名字为2.jpg的图片跟程序放在同一目录下

运行结果：

参考文章1：PyQt5中如何在lable中加载的图片上绘制矩形框呢？
参考文章2：PyQt5 在label显示的图片中绘制矩形的方法