Hi,
To replace the header module background (conference scene) please updload your image with the same size and name header_bg.jpg to templates/gk_events/images/header_bg.jpg (you can do it via your FTP client)
To change the logo via CSS, please add the following custom css code into your gk_events/css/override.css file (don't forget to enable override option from Template Options - > Advanced tab)
.svg #gkLogo.cssLogo, .svg #gkLogoSmall.cssLogo {
background-image: url(../images/path-to-your-image-logo);
}
Let us know if you need further help..
Cheers
Joshua