In the version 4.0 PHP templates, to change the members area header image simply upload your header image over the existing members header image via FTP to the following location:

/members/images/header/members_header.jpg

To replace the headers( we call them headers as banners are what advertisements are referred to ) you can simply right-click on the header image and get the location and name of the file on the server. Then use FTP to upload your new header image to the same location. If you use thew same name you can just upload your file over the existing one.

As an example, the header image for the members area is this:
http://domain-name.com/members/images/header/members_header.jpg
located on the server at:
/domain-name.com/public_html/members/images/header/members_header.jpg
The dimensions of the header image(as reported by FireFox's "View image info" right-click) is:
1,988px × 300px (scaled to 994px × 150px)
So if you create the header image at 1,988px × 300px it will scale it down to 994px × 150px on normal monitors but it will show the full resolution on displays that have high resolution.