Image help please

I’m new at this and I don’t have any experience, I’m pretty happy with my site the only thing is that I can’t change the image on the header of the page.
I’m using theme Atahualpa 3.4.1, and can’t figure out how to change the picture, I really need some advice,I have a folder with the image that I want to use but I can’t figure out how to put up.
My site is http://www.gevastation.com it may contain some NSFW content is not porn.

Thank you very much in advance.