how can i use a image as header background?

i am using the free Vantage. how can i use a image as header background? i have download Child theme, but can’t find the useful documents in editor. and i tried:

/* Masthead Background Image Cover */
header#masthead {
background-image: url(“http://www.maxin.com.au/wp-content/uploads/2015/10/flower-border-clipart-direyaKeT.png”);
}

but it is not working.

URL: http://www.maxin.com.au/