I found a workaround that works for me. Instead of just changing the wallpaper, or pressing "Save Preferences", I press "Quit Slidewall" and the new setting is saved to ~/.local/share/slidewall/config/livemode. When I restart the setting is retained.
Almost the same here, on ubuntu 13.10.
On re-boot Slidewalll starts with Live-Earth instead of the one I've choosed before saving preferences.