Full width Image

Hi
I am trying to insert an image after the header and make it full width to the page. Now it seems that I can either add this code to the functions.php file (which I do not want to do as I do not have a child theme running)

function after_head_image_func($content){
	echo "<div class='custom_content'><img src='_URL_'></div>";	
}
add_action('ava_after_main_container', 'after_head_image_func');

or perhaps make an image and combine the header and image and place it in the image.
I did use a slide but it comes in too slowly.
Any idea on that
Regards
Richard