Put custom size header image in Twenty Fourteen

I just switched over to Twenty Fourteen and I want to have a header image at the top of my site. It has to match these criteria:
1. be a background image (so that it isn’t “right-clickable” if that makes any sense).
2. be centered over the posts’ content
3. have an easily changed height and width.

My editing skills are ok – I probably could figure this thing out if I gave it some time and wasn’t so tired – but I’d really appreciate some help with this!

Thanks!