Transparent Menu Side Nav, Then Solid Image Background

Hi,

I’m using the side menu with “transparent” option on my site, thebutterflypavilion.com. I have a background image uploaded for when you scroll down the page, but the menu is defaulting to a grey background instead of the image. How can I make the menu transparent when the page first loads, then display an image once you scroll down, instead of the grey? Please advise.

Thanks,
Kelsey