Just further qualify that CSS statement with the menu item class on the body tag which takes the form "itemid-123" in Joomla where 123 is the menu item ID - there is something similar for WP too.
You didn't say which CMS you are using?
so in Joomla your statement would be:
.itemid-123 #g-navigation {
background-image: url("yourimageurlgoeshere");
background-repeat: no-repeat;
background-size: cover;
}
to affect a specific menu item with ID 123.
Regards, Mark.