How can I add a logo to this site that is stretches from one side of the header to the other.

This site will not allow me to upload a header image (set at 0px height) and the logo is apart of a 1/3 column and I can't figure out how to get it to reach end to end without jacking up the header. Here is what I'm going for compared to what's currently on the site.

http://5b7.60d.myftpupload.com/wp-content/uploads/2015/08/DodsonImage.png