How to draw a circle and extract the portion using the centroid of an image ?

I want extract an area of soil image using centroid of the image. Basically I want to find the centroid of an image from a folder of images and then using centroid find an area which will be used for extraction of features like colour moments,entropy and so on. The area can be a circle or square.And then how to use that extracted part (like circle) for feature exatrcion?

I would appreciate it if you could help me to solve this problem ! Thank you :)