Hi Paul,
One way of doing this is by adding a JS script that checks if the page has a header image, if it doesn’t then it adds one. Please include the script below in your theme’s functions.php file located at ‘wp-content/themes/your_theme/function.php’. Note that you can change the image URL by changing the value of the ‘background-image’ property in the snippet.
function default_header_js() { ?>
<script type="text/javascript">
jQuery('document').ready( function(){
if ( !jQuery('.page-header-overlay').length ) {
jQuery('.page-header').prepend('<div class="page-header-overlay" style="background-image:url(http://tomschaller.org/youth/wp-content/uploads/2015/01/YouthEvents.jpg);"></div>');
}
});
</script>
<?php
}
add_action('wp_head', 'default_header_js');
Please let me know if it does the trick,
Best,
Nico