Use Image Map in Header with Thesis

Hello!

I’m trying to use an image map as my header image which includes a custom navigation bar inside on it too. I’ve created the image, and the necessary html but I’m not sure where I need to insert that html. Do I need to go into FileZilla to do this or do I do it in wp-admin?

I use Thesis theme and the header.php file is not visible so I’m completely lost.

The site in question is https://www.beachblondeblog.com

Any help greatly appreciated! Thank you!