How to change the header

Instead of replacing the "logo" in the header, I would like to replace the header. It seems to be named "blogsmu_top_big.jpg" with the dimensions of 958x73 pixels. But, I can't seem to find the file location. Where is this jpg file located?

Also, is there a pdf file or a link to where I can find a Template layout and/or cheatsheet for making some typical customizations to this theme? There was no 'readme' file or such in the Theme's zip files.