Сверху
Код:

!define MUI_HEADERIMAGE
!define MUI_HEADERIMAGE_BITMAP "C:\Program Files\NSIS\Contrib\Graphics\Header\orange-uninstall-nsis.bmp"
Слева
Код:

!define MUI_WELCOMEFINISHPAGE_BITMAP "C:\Program Files\NSIS\Contrib\Graphics\Wizard\orange-uninstall.bmp"