Changing background on Platform start

Hi,
John T. Haller himself has explained it in this 2007 post: https://portableapps.com/comment/55200#comment-55200

It's built right into the PortableApps.com Platform starting with the 1.1 Beta Release. Place a file call portableapps_wallpaper.bmp in your Documents\Pictures directory and the menu will swap it for the Windows wallpaper while the menu is running and swap it back when you close it. Nice and easy. No extra software required.

And user ApexDestroyer has written in 2018: https://portableapps.com/comment/238553#comment-238553

Using The Built-In Wallpaper Swapper
The PortableApps.com Platform has an easy-to-use wallpaper swapper built in. It will change the wallpaper on the PC to your own while it's running and change it back when you exit. To use it, just save the wallpaper of your choice as a Windows BMP file (RGB encoding for the geeky) to your X:\Documents\Pictures directory and name the file portableapps_wallpaper.bmp. The next time you start up the PortableApps.com Platform, it will find it and use it. You can also add a portableapps_wallpaper_wide.bmp file which will be automatically be used on widescreen displays.

The John T. Halle's method worked fine, but I couldn't make ApexDestroyer's one (portableapps_wallpaper_wide.bmp) to work at all.

I hope it helps.