The Set Desktop Image action (v9.0+) allows you to set the desktop to a specified image file.
The image must remain in the file at that path since the system will potentially reload it (eg after a restart).
You can have the image:
Fill the screen, scaled large enough to fill the entire screen and crop in one dimension.
Fit to screen, scaled large enough to fit in the screen, leaving part of the screen as a background colo of your choice.
Stretch to fill the screen exactly.
Center unscaled on the screen, possibly leaving part of the screen as a background colo of your choice.
You can set all the screens, or a specific screen. Choices include:
All screens.
Main screen.
Second screen (leftmost non-Main screen).
External screen.
Internal screen.
Front window’s screen.
Back screen (leftmost non-Front window’s screen).
Screen with mouse.
Screen with a specific index (from left to right).