Header picture

Hello, we are creating web pages for my husband like portfolio (http://www.tomasjanousek.cz). To the header we need his photo. On the first page it looks good. But when we go to other page, it looks horrible. There is only part of the photo. It is only for desktop. When I try it on tablet or mobile, it looks good. Please let us know what to do, what to set and how to repair it. Thank you very much. Petra