Adding Images/Ads Next To Header Image?

Hi guys. I’m going to be offering ads soon, but I’m having difficulty placing ads at the top (in either of the corners) of the page.

Can someone advise how I can do this? In addition, my header image spans the whole width, and I’m not sure how to change this as making the source image smaller doesn’t seem to make a difference, and I want to retain the width of the rest of the page.

The blog is here: https://overratingtheunderrated.com

Thanks.

EDIT: To elaborate slightly, I’ve now figured out how to add an image to the header, but it appears above the header/logo, and I would like it next to it/on top of it.