I would like to know how I can make my header not scroll, rather stay fixed for the Superhero theme?
I checked http://femicideacuns.wordpress.com/ and I see that you figure this out using the following CSS:
Also, if it's possible to extend the image in my current header as the full background of the header. (if that makes sense.)
Here is some CSS that will add a repeating image all the way across the top of the main header in the Superhero theme:
If you make a change like this, you will probably also want to edit your font colors so they're easier to read. Try modifying colors using the Appearance → Themes → Customize → Colors panel first.
And you can either change out the existing background image or remove it from the Appearance → Themes → Customize → Background panel. Click the arrow to the right of the image there to see the options.