What do these properties mean in background size drop-downs?

Can you tell me in the “background size” drop-downs what the properties None, Cover, Contain, and Flex do to the related image. I understand the principle, but how does it work in your plugin? Sometimes my logo image changes from the original size when I change this property. So it’s confusing to us newbies. Thanks again.

http://wordpress.org/extend/plugins/custom-login/