Placing the Featured Image Behind Navigation Bar

Hi there,

On all of my website I managed to put the featured image behind the transparent navigation bar at the top, which I love. However, on my WooCommerce shop site it somehow refuses and the image gets placed beneath the navigation bar, leaving the navigation bar looking awkward and invisible with white font.

How can I force the featured image to go all the way up and behind the navigation bar?

Any help would be greatly appreciated! Thank you.