If you take focus from the wizard and drag other windows on top of it, something turns the header image bitmap visible on the welcome page.
This happens and is reproducible on other systems (I have tested it), however it is an occasional occurrence so it might not be apparent on first try.
1. NSIS version 2.46 on Windows 7 32-bit
3. Any script with MUI or MUI2 that provides a custom header image