If what you need is the image to have the same dimensions of the div, I think this is the most elegant solution:

background-size: 100% 100%;

If not, the answer by @grc is the most appropriated one.

Source:

You can use this attributes:

background-size: contain;
background-repeat: no-repeat;

and you code is then like this:

 <div style="text-align:center;background-image: url(/media/img_1_bg.jpg); background-size: contain;
background-repeat: no-repeat;" id="mainpage">

background-position-x: center;
background-position-y: center;

you also use this:

background-size:contain;
height: 0;
width: 100%;
padding-top: 66,64%; 

I don't know your div-values, but let's assume you've got those.

height: auto;
max-width: 600px;

Again, those are just random numbers. It could quite hard to make the background-image (if you would want to) with a fixed width for the div, so better use max-width. And actually it isn't complicated to fill a div with an background-image, just make sure you style the parent element the right way, so the image has a place it can go into.

Chris

try any of the following,

background-size: contain;
background-size: cover;
background-size: 100%;

.container{
    background-size: 100%;
}

Alternative:

background-size: auto 100%;