Changing header on Thesis theme

I have 4 websites that were set up for me in 2011. I want to change the header image. On three of the sites Thesis set up options is in the side bar and I can see where to change the image. But on the 4th for some unknown reason the Thesis options are not in the side bar.

https://www.egyptpropertyforsale.co.uk/

So I thought as a last resort I could FTP a new image across.

Can someone tell me where I can find out where the heck the header image is stored so I can overwrite it. (Yes I will take a back up first)