menu dropdown image background

Navigate to YOOtheme > SETTINGS > Custom Code > CSS/LESS, and add the following to it:

.uk-drop.uk-navbar-dropdown {
background: transparent url(../../../images/submenubackground.png) no-repeat;
background-size: contain;
top: 0px !important;
padding-top: @navbar-nav-item-height;
/* optional, depending on the style */
border: none;
box-shadow: none;
}

To adjust a possibly suitable shift to the lef — depending on the concept and the subject of the actually used background image — use the parameter YOOtheme > STYLE > Components > Navbar > Dropdown > @M Shift Margin, and set it to -80px, for example.

Layout / Header / Dropdown - change to center to line the image up in the centre of the main menu item, then...

play with the width of the dropdown in styles / navbar to get the image to centre.