Advert on front page

I know this rather goes against the whole minimalism of the theme but I’d like to add an advert at the top of the home page, beneath the banner. Well, I’ve done this except it’s centred to the page, meaning that the banner overlaps the vertical line and is at odds with the posts listed underneath.

Can anybody suggest a way to add a banner so that it aligns, maybe the left-hand starting to the right of the vertical line, as per the posts?

Thanks in advance,
David.