Header Enlargement

I have a CatchBox Pro Account. My header is 143 pixels tall if I remove the heading and zero out the margins. Recently I acquired some artwork that I want to incorporate into the site. I'd like to increase the height of my header to exceed 143 pixels so that I can use a larger image in the header field. How do I do that? My website is www.restoringpangea.com Thank you for your help. Nathan