The ideal image ratio should be 1:1, for example, 660x660 pixel or 1500x1500 pixel. The file should be a jpg, png, or gif.
If the uploaded image is within 20% of the 1:1 ratio, for example, it is 440x680 pixels (the image is horizontal), it will be cropped from the top and bottom to fit the target ratio (660x660). If the opposite situation happened and the image was 680x440 pixels (the image is vertical), then it will be cropped from the sides to fit the target ratio (660x660).
If the image is not even close to the target ratio (e.g., more than 20% should be cropped), for example, 1200x440, the image will get cropped from all sides to fit the 1:1 ratio.
What tool to use to resize and crop images
You can use a basic image editor (like Paint) or some online tools like Cropp.me or Cut My Pic.