Backgrounds shouldn't remember settings for unused workspaces by default.

[Starcrasher]

If you set background parameters for a workspace, then stop using it by reducing number of workspaces shown. The settings for that unused workspace are remembered, at least until reboot.
I would expect these parameters to take back default values as soon as the number of workspaces is reduced.
Remembering them might be optionnal, for people who really want it.
Number of workspaces is determined in Screen, the values are set in Backgrounds. I hope I chose right component...