Well... that undermines caching, probably can be achieved by on the cost of performance. Or if no previewIMage is set - after you clean cache use next random image (right now 1st image is used)
Ok, I've figured it out. Go to admin settings then scroll down to menu/mainmenu/folderdescription. Uncheck enabled then save. Then check enabled and control click which items you want to enable. Click save. Finished!