Manage > Customize, Customize your theme (right hand side)
Look in your Custom CSS (left hand side, down a bit) and see if there's an image name that corrolates to your header (copy/paste every URL that looks like an image, LOL), and if you can find it, make your own header, upload, and then substitute the URL for YOUR image for the one already there.
It's something to try. If it's not in your Custom CSS I have no clue, LOL.
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Look in your Custom CSS (left hand side, down a bit) and see if there's an image name that corrolates to your header (copy/paste every URL that looks like an image, LOL), and if you can find it, make your own header, upload, and then substitute the URL for YOUR image for the one already there.
It's something to try. If it's not in your Custom CSS I have no clue, LOL.
Good luck! It's an awesome layout.