I've yet to see a known-good way of using a different image between different nav menu elements, though I know there is a way to use one image as the consistent separator.
I have seven nav elements:
#menu-item-26{
}
//blue square here
#menu-item-25{
}
//yellow square here
#menu-item-24{
}
//red square here
#menu-item-23{
}
//pink square here
#menu-item-22{
}
//green square here
#menu-item-21{
}
//purple square here
#menu-item-20{
}
And I've created 6 images in photoshop, essentially just 10px by 10px squares, each a different color. I want to use a different one between each set but I don't know if it would need to be used as a background image with each item and padding or if maybe there's another responsive-friendly way.