Responsive Nav Toggle Image

I’m using on a menu for a site. You can see it here:

When the browser is narrowed and the nav collapses it becomes a toggle with the word “Menu”. I’d like to have an image there instead.

Currently in the responsive-nav.css I have an image assigned to the background of #nav-toggle. It is only partial showing and I can’t figure out how to make it all appear. I think that is not the correct way to do this.

In the responsive-nav.js file there is a section of customizable options including the one for the label:
label: “Menu”, // String: Label for the navigation toggle

That word "Menu is showing on top of the small bit of background-image I have assigned to #nav-toggle. If I delete the word “Menu” from the .js option then nothing appears at all for the toggle.

Is there a way to replace the word “Menu” in the .js file with an image? I don’t know JavaScript so I’m not sure if this is possible or what the code would bne to do that.

Any advice on how to have a custom image versus the word “Menu” for the toggle will be much appreciated. thanks in advance for any help.