How can i have a banner ?

Hello and in advence sorry for my english and my poor skill in making a website.

This is my site http://www.vieuxetmechants.com

I have a problem with the logo. It’s too small. I wish i can have the place to put a banner like this http://hpics.li/4e0f13c. I understand it can be too big and i must resize the banner.

So my question at last : How can i modified the space for the logo ?

Thanks