100% width background image with an 'auto'

Question

I'm currently working on a mobile landing page for a company. It's a really basic layout but below the header there's an image of a product which will always be 100% width (the design shows it always going from edge to edge). Depending on the width of the screen the height of the image will obviously adjust accordingly. I originally did this with an img (with a CSS width of 100%) and it worked great but I've realised that I'd like to use media queries to serve different images based on different resolutions - let's say a small, medium and a large version of the same image, for example. I know you can't change the img src with CSS so I figured I should be using a CSS background for the image as opposed to an img tag in the HTML.

I can't seem to get this working properly as the div with the background image needs both a width and a height to show the background. I can obviously use 'width: 100%' but what do I use for the height? I can put a random fixed height like 150px and then I can see the top 150px of the image but this isn't the solution as there isn't a fixed height. I had a play and found that once there is a height (tested with 150px) I can use 'background-size: 100%' to fit the image in the div correctly. I can use the more recent CSS3 for this project as it's aimed solely at mobile.

I've added a rough example below. Please excuse the inline styles but I wanted to give a basic example to try and make my question a little clearer.

<div id="image-container">
    <div id="image" style="background: url(image.jpg) no-repeat; width: 100%; height: 150px; background-size: 100%;"></div>
</div>

Do I maybe have to give the container div a percentage height based on the whole page or am I looking at this completely wrong?

Also, do you think CSS backgrounds are the best way to do this? Maybe there's a technique which serves different img tags based on device/screen width. The general idea is that the landing page template will be used numerous times with different product images so I need to make sure I develop this the best way possible.

I apologise is this is a little long-winded but I'm back and forth from this project to the next so I'd like to get this little thing done. Thanks in advance for your time, it's much appreciated. Regards

Answer 1

Tim S. was much closer to a "correct" answer then the currently accepted one. If you want to have a 100% width, variable height background image done with CSS, instead of using cover (which will allow the image to extend out from the sides) or contain (which does not allow the image to extend out at all), just set the CSS like so:

body {
    background-image: url(img.jpg);
    background-position: center top;
    background-size: 100% auto;
}

This will set your background image to 100% width and allow the height to overflow. Now you can use media queries to swap out that image instead of relying on JavaScript.

EDIT: I just realized (3 months later) that you probably don't want the image to overflow; you seem to want the container element to resize based on it's background-image (to preserve it's aspect ratio), which is not possible with CSS as far as I know.

Hopefully soon you'll be able to use the new srcset attribute on the img element. If you want to use img elements now, the currently accepted answer is probably best.

However, you can create a responsive background-image element with a constant aspect ratio using purely CSS. To do this, you set the height to 0 and set the padding-bottom to a percentage of the element's own width, like so:

.foo {
    height: 0;
    padding: 0; /* remove any pre-existing padding, just in case */
    padding-bottom: 75%; /* for a 4:3 aspect ratio */
    background-image: url(foo.png);
    background-position: center center;
    background-size: 100%;
    background-repeat: no-repeat;
}

In order to use different aspect ratios, divide the height of the original image by it's own width, and multiply by 100 to get the percentage value. This works because padding percentage is always calculated based on width, even if it's vertical padding.

Answer 2

You can use the CSS property background-size and set it to cover or contain, depending your preference. Cover will cover the window entirely, while contain will make one side fit the window thus not covering the entire page (unless the aspect ratio of the screen is equal to the image).

Please note that this is a CSS3 property. In older browsers, this property is ignored. Alternatively, you can use javascript to change the CSS settings depending on the window size, but this isn't preferred.

body {
    background-image: url(image.jpg); /* image */
    background-position: center;      /* center the image */
    background-size: cover;           /* cover the entire window */
}