As I have read so far, it seems that the header image of a wizard page created by NSIS MUI can be only left or right aligned?
Is there any possibility of centering the image?
You can do whatever you want with it—but you will need to modify the dialog, which is stored in a resource in an executable. Take a look in the Contrib\UIs directory. You'll need a resource hacker to change it (modern.exe is the default, see Contrib\Modern UI\System.nsh). Then when you're done you can define MUI_UI to the path to your modified executable.
Also consider how you'll get the header text in if you centre the image (or if you want to throw it away).