Defines the position of the background image.
默认属性
background-position: 0% 0%;
The background image will be positioned at 0% on the horizontal axis and 0% on the vertical axis, which means the top left corner of the element.
background-position: bottom right;
You can use a combination of position keywords: center, top, bottom, left and right.
background-position: center center;
The background image will be positioned in the center of the element.