CSS Background Size

The background-size property enables one to control the scaling of the background-image. It takes up to two values, which determine the scale/size of the resulting image in vertical and and horizontal direction. If the property is missing, its deemed auto in both width and height.

auto will keep the image's aspect ratio, if it can be determined. The height is optional and can be considered auto. Therefore, on a 256 px × 256 px image, all the following background-size settings would yield an image with height and width of 50 px:

A background-size has the following syntax

background-size: 50px;
background-size: 50px auto; 
background-size: auto 50px;
background-size: 50px 50px;

contain

Scale the image, while preserving its intrinsic aspect ratio (if any), to the largest size such that both its width and its height can fit inside the background positioning area.

This makes sure that the background image is always completely contained in the background positioning area, however, there could be some empty space filled with your background-color in this case:

background-size: contain;

cover

Scale the image, while preserving its intrinsic aspect ratio (if any), to the smallest size such that both its width and its height can completely cover the background positioning area.

This makes sure that the background image is covering everything. There will be no visible background-color, however depending on the screen's ratio a great part of your image could be cut off:

background-size: cover;