You can download the wallpaper itself here. (This URL points to the wallpaper of the current Squeak release, but you ought to have at least 6 months from today before it changes :) )
Wallpaper in hand, and assuming that the image is in your current directory, you need just evaluate Form openAsBackground: (FileDirectory default / 'wallpaper.png') pathName
.
(I should add that the background image is intentionally not part of the update stream: it would bloat images, and change people's existing backgrounds. Thus, it's applied (along with a few other tweaks) "on the side" by the release process.)