ROI for image dataset - python

I'm trying to find out ROI for an image dataset. I'm able to find ROI for a single image, but when it comes to the whole dataset or multiple images I can't do it.
I tried to use different code from the internet, but nothing worked.
`
import cv2
import numpy as np
import os
import random
from matplotlib import pyplot as plt
import matplotlib.image as mpimg
img_folder=r'Shrimp/train/Resized_Dataset'
img_dir = r'Cropped_shrimp'
for i in range(30):
file = random.choice(os.listdir(img_folder))
image_path= os.path.join(img_folder, file)
img=mpimg.imread(image_path)
ax=plt.subplot(1,30,i+1)
ax.title.set_text(file)
plt.imshow(img)
roi = cv2.selectROI(img_folder)
ROI_image = img_folder[int(r[1]):int(r[1]+r[3]),
int(r[0]):int(r[0]+r[2])]
cv2.imshow("ROI_image", ROI_image)
cv2.waitKey(0)
os.chdir(img_dir)
cv2.imwrite("ROI_image", JPG)
`
This is my last approach. I think there're lots of mistake because I'm trying this for the first time.

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