Hello,
1) If you want to add the background image for the cart page title you can use custom code for this page only because there is no special option, for example, http://prntscr.com/mu6lft
.cart-checkout-nav { background-image: url(https://www.leshopdescoquettes.fr/wp-content/uploads/2019/03/Patricia-3.png); }
2) You did not add the units for the height http://prntscr.com/mu6jvs
Regards
