Rebranding WPMU - manually edit image?

I am keen on using the Rebranding WPMU plugin and after describing the installation process the document reads, -

"Once you’ve done that you need to manually edit the following image files (naturally we can’t make your logo for you):

wp-admin/images/logo.gif
wp-admin/images/logo-ghost.png
wp-admin/images/wordpress-logo.png
wp-admin/images/wp-logo.gif
wp-includes/images/wlw/wp-comments.png
wp-includes/images/wlw/wp-watermark.png
wp-includes/images/wlw/wp-icon.png
wp-includes/wordpress-admin-bar/themes/grey/images/background.png
wp-includes/wordpress-admin-bar/themes/grey/images/background-logo.png
wp-includes/wordpress-admin-bar/themes/grey/images/bullet_arrow_down.gif
and maybe wp-includes/wordpress-admin-bar/themes/grey/grey.css"

I did not understand what I need to manually edit these images/files for. Can someone please explain? ( It seems that the login image is replaced once I upload an appropriate image. Can other wordpress logos be replaced by this plugin too? I may be repeating myself here, but what is the editing required for?)