Yeah, I agree my friend. Just thinking of when you (and I) was yelling at the screen to ‘drag the image!’ How could the user be helped to discover this ability?
Although now that I look, it is mentioned in the user guide!
If you are using an image, you have to decide on the placement:
Manual lets you specify the coordinates. You can drag the picture around in the preview to the left or enter X and Y manually.
https://www.haiku-os.org/docs/userguide/en/preferences/backgrounds.html