Creating a full width header in Default themes

Hello just looking for a few answers. I'm pretty familiar with CSS and HTML but still learning WP. So I have a few questions 1. What is the best way to create a full width header for a default theme like TwentyTwelve or TwentyThirteen? I understand some of the basics, i.e., create child theme and edit css in style.css. But having mixed results.
When I look at the theme TwentyThirteen it almost has a full width header (some space on both sides but very close). I looked at the themes default images Star.png and Diamond.png in the images/header files and notice that they come in a size that is 3200 x 420px however if you go into the dashboard of the theme and upload your own header you are asked for an image that is 1600x230px and if you upload an image that size it's not nearly as full as the Diamond or Star.png that ship with the theme. I do know that WP resizes and creates different sized images, but wondering how this works?

I downloaded Star.png and modified it, then uploaded it to the folder and named it Star.png and sure enough it works. But if I wanted to do this in a child theme would I need to create the folders for the header images in the child theme? Why the difference in image sizes? What function controls this and could I add another image with a name different than star or diamond.png which would allow you to choose it from the dashboard? Thanks