Using full width header option

I would like to span my header image across the page similar to this website w w w.unit4.c o m

I have used the style options and selected Full Width Header - yes

However it moves to one side.

So I have a couple of questions

What size image should I be using - I have read that it should be the size of the max screen resolution you expect the site to be looked on. However all the ones I have looked at are 1000px but span the whole screen at 1280px

I've tried centering the header image but this doesnt seem to work either.

I dont have an active site, just trialling on my local machine at the moment