responsive background image in header

I want to add a responsive background image to the #header-inner-bg
I’ve added code to css style sheet:
#header-inner-bg {
background-image: url(http://localhost:8888/wp-content/uploads/SPTHeaderbg.png)!important;
background-position: center center;
background-repeat: no-repeat;
}
My image size is 1240 x 386
How can I make this image responsive — it’s not stretching to max 1240 size, nor is it responsive on mobile size?