There is a tutorial on Getting Started with Images.

import numpy as np
import cv2

# Load an color image in grayscale
img = cv2.imread('/path_to_image/messi5.jpg',0)

# Show the image
cv2.imshow('image',img)
cv2.waitKey(0)
cv2.destroyAllWindows()

Use an absolute path to the image, and then you don't have any path problems.

Absolute and relative paths

OpenCV Error: (-215)size.width>0 && size.height>0 in function imshow

cap.read() returns a tuple, which contains a boolean success flag, and the video frame.

if the video has ended, the success flag is False. in that case, the frame is empty.

you MUST test that flag and end the loop.

the tutorial you watched failed to do so. it taught you wrong.

in addition, you must check if cap.isOpened() because opening the video file (or camera) could have failed, and you need to react to that.

here is the proper code:

cap = cv2.VideoCapture(...) # open a video file or camera
assert cap.isOpened(), "file/camera could not be opened!"
while True:
   (success, img) = cap.read() # cap.read() always returns a tuple of two things
   if not success: break # you absolutely must check this
   cv2.imshow("Video", img)
   if cv2.waitKey(1) & 0xFF == ord('q'):
      break

When you put cap.isOpened() it check that, the video is read properly, So the while loop is not working there.

But when you changed to while True it will execute without proper reading that's why it's giving an error.

Make sure than you are properly reading the video file.