Changing Banner/Header Image?

Ok, first of all I am using version 3.3.1. Second, I am completely new to programming, so please be simple. I normally am in charge of submitting content, but the banner needs to be changed and our IT guy is being lazy like always. The image is already done, it was drawn in Gimp/Blender and I believe it’s a JPEG file.. So what are the first steps in changing a banner/header? I’ve read articles about going to Header.php, I went there, and couldn’t read anything, and I think I remember my IT guy saying not to change it like that. Either way, please help!
Thanks