Header area making it larger

https://www.seehowtomakemoney.net

The current theme is by Woothemes. I am trying to make the dark header area larger so that I can fit in my logo without having to make my logo any smaller. (its looks better bigger) I posted on there support and they linked me to how I have to take that image (assuming form ftp?) and enlarge it in photoshop. I have questions on how to do that though.

1) I dont know where the image would be found on my server.
2) what dimensions would it be more specifically the horizontal pixel size?

Regards
-kevin