Difficulty applying still wallpaper over a Dreamscape

My PC came with Vista though I switched to XP and Dreamscapes run OK on it.

But nothing happens when I try applying ‘set as desktop background’ for ordinary wallpaper. It won’t replace the animated theme, which keeps running.

I can’t find any process in Windows task manager that looks like an MPEG-2 video decoder, and I’m not sure if killing it would be the right thing to do.

I haven’t tried switching from whatever WB theme I’m using back to Windows Classic or XP theme, then reapplying the WB theme and still wallpaper, so I don’t know if that’ll do the job.

But if it does, it sure seems a long workaround for what is probably a straightforward way of applying still over animated paper.

Is there a direct way of switching between still and animated I’m missing?

Appreciate your help, thanks.