Can't overwrite parent circles.png in twentythirteen with my child theme

I’ve created a child theme and linked it successfully to the parent theme.

I can change the colors of fonts.

However, I can’t overwrite the circles.png header image with a new image. In the twentythirteen theme.

I don’t understand how one element can be overwritten with a child theme and another element can not.

As a matter of fact, when I look at the page in Chome dev tools. My new image in actually loading BEHIND the circles.png twentythirteen image.

https://brewhahamedia.com/wordpress/

I’m using this CSS in my child theme style.css file –

.site-header {
background: url(https://brewhahamedia.com/wordpress/wp-content/themes/twentythirteen-child/images/cats.jpg);
}

let me know what other info would be of help.

Thanks in advance