img1 = cv2.imread('Tarun.jpg',0)
As you know, imread reads in the specified image. With just the filename, as here, python tries to read the image from the current working directory. If the file does not exist, no error is reported and img1 will have a value of None.
That this line:
cv2.imshow('Hey its me Tarun yellogi',img1)
returns an error of:
error: C:\projects\opencv-python\opencv\modules\highgui\src\window.cpp:331: error: (-215) size.width>0 && size.height>0 in function cv::imshow
means that the image specified in the imshow command (i.e. img1) has a width and/or a height of 0. What the error message is saying is that the image in function imshow must have a size where the width is greater than zero and the height is greater than zero.
You can check the image size with img1.shape. In this case you will probably get an error message AttributeError: 'NoneType' object has no attribute 'shape'. You can further check if img1 is None with img1 == None. If you get a response of True then you will know that img1 does not contain image data.
TL;DR
The file 'Tarun.jpg' was not found thus img1 == None.