This doesn't work for me.
This is what I hate about KDE4. There are so many different ways of doing anything, and what applies one place doesn't work in another, altho it seems that what you are trying to do is allowed, but then it doesn't work. Then there is the law of unintended consequences, what you just did screws something else up and you don't realize it for a while, then you screw something else up trying to fix the first screw up, the before you know it EVERYTHING is screwed up. And just doing away with desktop effects, doesn't get things back to SIMPLE
Right now my desktop has no ability to have different wallpapers on different desktops, yet different activities is checked, which always enabled different wallpapers before.
On my desktop, I can grab a white space and drag the wallpaper around, showing other wallpapers that I have used. telling it to scale, scale cropped, etc has no effect on this, tho I thought it did at one time.