Thanks guys. I did as you said and it works.

I really thought it's temporarily inactive in this version of MorphOS. Before I've posted this question I specified a drawer containing my wallpapers and then removed the picture filename from the File: textfield and the slider didn't get activated. That's why.
Right after I've read your replies I clicked the File: field activated and hit Return on keyboard. And that was the key detail -> hit Return

Previously I just moved the focus elsewhere after removing the picture filename.. That should be enough to activate random background IMHO, but it isn't.
Don't you think?