slackbuilds/desktop/xwallpaper

The xwallpaper utility allows you to set image files as your X
wallpaper.  JPEG, PNG, and XPM file formats are supported, all
of them being configurable and therefore no fixed dependencies.

The wallpaper is also advertised to programs which support
semi-transparent backgrounds.

A zsh function is installed to /usr/share/zsh/site-functions.