Given a square image, how can I crop, at the exact center, of the image?

What if the image is not square itself?

What if I'm using Photoshop CS6 or some older version? 
Is the procedure the same?


Question: Enabling aggregate and compress CSS files caused error on site access
Answer: If you're looking for a good alternative for core's preprocessing of CSS/JS check out the Advanced CSS/JS Aggregation module