how to know the position of pixels in a picture

Hi, please , i ask this query as i want to do some video edit on a video.
Now i have taken a snapshot of a video.
Now, i have to know the position of starting of a rectangle width and height to issue a ffmpeg command
For that , i need the position of start of upper part of the video
how to know the pixel position as it is very difficult to find
i have tried microsoft pic manager, it helps in resizing the picture.
i will try to know these things from an editior
But is there any easy way of knowing the pixel position.
The deduction of cropped portion in picture manager would cut the left and right most part of the video as well.
experts please