How to properly edit your header?

[Colombo]

Hi all,

I am at my wits end when it comes to this problem, and all of the solutions I've found on google involve editing HTML code which simply doesn't appear in my template.

I can't post a link yet but my blog's name is dissectingthegame

I replaced the header with my own custom image as you can see. However, despite the fact that I checked off the box 'instead of title and description' while uploading my image, the old, plain brown header pokes out all around the edges of my custom image no matter how big I make my image. How can I remove the old header altogether and just have my custom image?

I also have the same problem when it comes to widget boxes not being used (see the very bottom of my blog).

Thanks for the help! Much obliged