Can someone help me with my new blog?

I just started a blog called Internet Vitamins on tumblr and im having some trouble. I can’t quite replace the header with an image. I’m not quite good with HTML so im not exactly sure how to do it. I tried following this guys instructions that helped me in the past but no luck. The layout code is a bit too long so i can PM you if you’re interested in helping me or post in the comments as a reply. Any help is extremely appreciated. Thank you

By the way, the theme’s name is Ultralite