Cannot set wallpaper on Ubuntu 15.04

[micooz]

[2016-03-07 08:57:53.647] [INFO] wallpaper - the current platform is: linux
[2016-03-07 08:57:53.701] [INFO] wallpaper - fetching from: http://cn.bing.com/HPImageArchive.aspx
[2016-03-07 08:57:53.866] [INFO] wallpaper - downloading from: http://s.cn.bing.net/az/hprichbg/rb/CapeSebastian_ZH-CN9469145123_1920x1080.jpg
[2016-03-07 08:57:53.997] [INFO] wallpaper - setting wallpaper:  /tmp/tmp.jpg
[2016-03-07 08:57:53.997] [INFO] wallpaper - try: gsettings set org.gnome.desktop.background picture-uri file:///tmp/tmp.jpg
[2016-03-07 08:57:54.001] [INFO] wallpaper - try: gconftool-2 --set /desktop/gnome/background/picture_filename --type string /tmp/tmp.jpg
[2016-03-07 08:57:54.004] [INFO] wallpaper - try: /usr/lib/node_modules/wallpaper-js/lib/bridge/xfce.sh /tmp/tmp.jpg
[2016-03-07 08:57:54.005] [INFO] wallpaper - try: dcop kdesktop KBackgroundIface setWallpaper /tmp/tmp.jpg 1
[2016-03-07 08:57:54.008] [INFO] wallpaper - try: dconf write /org/mate/desktop/background/picture-filename "/tmp/tmp.jpg"
[2016-03-07 08:57:54.010] [INFO] wallpaper - try: feh --bg-scale /tmp/tmp.jpg
[2016-03-07 08:57:54.014] [INFO] wallpaper - try: pcmanfm -w /tmp/tmp.jpg
[2016-03-07 08:57:54.016] [FATAL] wallpaper - no proper method found to set your wallpaper

[micooz]

it works well these days, but I don't know why...