Hi,
there are several things to do.
1) Create your own background for the top with the same width of the template
2) Once you have done, insert it in templates/gk_blakandwhite/images/
3) After go to: templates/gk_blakandwhite/css/template.css and open it with a css editor
4) Search for "#gk-top" around line 50 and here insert you background with "background: url('../images/example.jpg') no-repeat;"
5) Modify/Create your logo and remove the black background of the logo. To do it, go to templates/gk_blakandwhite/css/template.css, search "h1.logo" and comment or remove the following: "background:transparent url('../images/logo_bg.png') no-repeat 0 0;"
6) Finally, if you want to remove the top border of the body, go to "templates/gk_blakandwhite/css/style1.css", search for "#gk-wrapper-content2" and replace "border: 1px solid #e0dfd5;" with "border-left: 1px solid #e0dfd5; border-right: 1px solid #e0dfd5; border-bottom: 1px solid #e0dfd5;"
I hope I did not forget anything...
Sorry for my English.
Regards