Background image cover depending on height

This might not be a support question but anyways.
I have a dropdown menu from top.

The problem is the background image is not covering whole menu.
It only drops down 50% so i only see half of the image in the menu or maybe less if i use lower resolution.

Can someone help with the right css code to use so i allways see full image no matter the size of the responsive menu?

#responsive-menu-container
{
height:50%;
margin-left:-8px;
}