Adaptive image in header

UPDATE: I think I may have figured it out. I don’t know if this will break something else, but it seems to work for me for now. Any feedback is appreciated.

In the WordPress theme editor, I added the following line to “Theme Functions (functions.php)” in the “Add custom header support” section:

background-size: 100% 100%;

I added it directly below:

background: url(<?php header_image(); ?>);

My new functions.php custom header support section looks like:

/**
* Add custom header support
*/
if ( ! function_exists( 'bouquet_header_style' ) ) :
/**
 * Styles the header image and text displayed on the blog
 *
 */
function bouquet_header_style() {
	// If no custom options for text are set, let's bail
	if ( HEADER_TEXTCOLOR == get_header_textcolor() && '' == get_header_image() )
		return;
	// If we get this far, we have custom styles. Let's do this.
	?>
	<style type="text/css">
	<?php
		// Do we have a custom header image?
		if ( '' != get_header_image() ) :
	?>
		#branding {
			background: url(<?php header_image(); ?>);
                        background-size: 100% 100%;
		}
	<?php
		endif;

Again, I don’t know if this affects other things in a negative way, but it fixed the problem of the header image not scaling.

Feedback is appreciated.

I am having the same problem. I tried this fix and it squeezes the header image horizontally so it becomes unreadable. If I use
background-size 100%, it squeezes the header horizontally AND vertically, and repeats. Instead of trying a no-repeat, I want to use a different image for 450px. So, in CSS. I have this:

@media (max-width: 450px) {
	#branding{ background-image: url(http://undertheboom.com/wp-content/uploads/2013/09/underboomphone2.jpg);
	}
	#branding .header-link {
		min-height: 90px;
	}
etc. }

My image is 450 x 90px.

But it does not work. The default header image gets cut off, and the smaller image does not load. I’m a novice, learning quickly, so it could be a missing bracket issue :-), but I can’t see it. Here’s the site: undertheboom.com

Thanks for any help!

Kay