Many thanks, Nikolay.
You got me started but it seems a more complex procedure than it really should?
Goal: An image 1920 X 1080 px for computer wallpaper with a monitor of that native resolution.
With the image open:
1/ Use the Resample tool with the target set to 1920 px in width with Preserve Aspect Ratio box checked.
2/ On image resulting from step 1, use the Dynamic Crop tool to produce an image 1080 px in height. Place the box where you want it and execute the process.
Is there no way to tell PI to put a crop box 1920 by 1080 onto your image so you can slide it about until you have the framing you want and then execute to produce the 1920X1080 image framed as you like?