Hello guys!
I wonder if the option "Change backround after:" is for changing ambient's background randomly.
How to active it?
ambient_background.jpg

Select a directory with pictures and let the field File empty, after that the slider will be adjustable

Just remove the filename and it'll show random pictures from the defined drawer.

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?