This thread提供了一般的解释：
GDI+ spins up a background thread as soon as you start manipulating images. It’s name doesn’t provide much of a hint what it does,“BackgroundThreadProc()”. This is pretty common in Microsoft libraries. Some sort of internal management thread,probably has something to do with memory management or threading. It creates a hidden window,that’s classic too.
Nothing to worry about,everything is normal. Including the exit code.
FWIW,this thread建议禁止背景线程：
If you do not suppress the background GDI+ thread,DDE commands can be prematurely issued to the application before its main window has been created. The DDE commands issued by the shell can be prematurely aborted,resulting in error messages.
