Header size and ads

My website is vinylmationkingdom.com and I want to thank you first for this theme. It’s the best one I’ve found. Love it.

Anyways, if you look at my website, I want to use a header that’s only about 400 pixels wide so that I can have an ad to the right of it? How can I do this? Everytime I upload an image that isn’t as wide as the site, the image is enlarged to the width.

Thanks in advance!!