BuddyPress.org

In the theme that you are running look for the file called screen.css

in it you can find the header section something that looks like:

/*** Header ************************************************************/

#header {

overflow: auto;

width: 100%;

background: #fff url(../images/LogoBanner.png) top left no-repeat;

}

it is a background image that points to a file in the images folder.

I am sure you can take it from there.