Don't feel stupid at all! If you scroll really far into some of the old threads about the layouts, you'll find me begging for someone to help. It took me FOREVER to wrap my brain around the whole "padding" thing. Like literally I could not figure it out for some reason! Haha
Here is a thread dedicated to layout/layout problems! You can find pre-made layouts that are awesome, and there are even some request threads somewhere in there. I'm sure someone would be willing to walk you through the steps of adding a banner; I would do it myself, but I'm honestly not very good at trying to help people with things like this -_- Honestly, though, it's not very hard to do.
Basically, when you're making your layout, go to the section that says "story title", and copy/paste the url to the picture that you'd like to use for your banner into the "background" space. After a few seconds (if your internet connection is decent) you should be able to see a little sliver of the photo in the title section of the layout. After you copy/paste, scroll down to the padding slider (and there are other sliders below it, but I can't remember the names of them XD) and adjust them until you can see the image completely. It's helpful to change the position of the image to "center" and take it off of "repeat" before you adjust the padding, as you'll be able to make it centered and whatnot.