Is there a way to set different wallpaper in different workspaces?

Here is one way. maybe not the best, but it works.

Add this line to ~/.fluxbox/keys file (change jpg to png if you use png wallpapers.)

ChangeWorkspace :Exec fbsetbg ~/.fluxbox/bg$(xprop -root _NET_CURRENT_DESKTOP | awk '{print $3}').jpg

In ~/.fluxbox symlink the wallpapers you want to appear on each workspace and rename bg0, bg1, bg2, bg3, bg4 etc

In fluxbox menu under fluxbox settings, reconfigure (may need restart as well)

OK-not sure about this bit;

In ~/.fluxbox symlink the wallpapers you want to appear on each workspace and rename bg0, bg1, bg2, bg3, bg4 etc

Thanks in advance for any advice.

For example, using rox-filer, drag the wallpaper you want from Wallpaper into ~.fluxbox and instead of clicking copt, click link (relative).
Once you have done this, then rename the ones in ~.fluxbox bg0 etc. bg0=workspace alpha.

Got it!
Thanks