Cannot change background image

[amreo]

I cannot change the background image of my desktop (wallpaper). I don't know the cause of the problem but I suspect the sharing of /home between ubuntu-16.04 and ubuntu-14.04 (Both installed on my laptop)
$ unity-control-center

(unity-control-center:1007): unity-control-center-WARNING **: Could not load /usr/share/themes/Adwaita/index.theme: File o directory non esistente

(unity-control-center:1007): Gtk-CRITICAL **: gtk_builder_get_object: assertion 'GTK_IS_BUILDER (builder)' failed

(unity-control-center:1007): Gtk-CRITICAL **: gtk_icon_view_get_selected_items: assertion 'GTK_IS_ICON_VIEW (icon_view)' failed

ProblemType: Bug
DistroRelease: Ubuntu 16.04
Package: xorg 1:7.7+13ubuntu3
ProcVersionSignature: Ubuntu 4.4.0-22.40-generic 4.4.8
Uname: Linux 4.4.0-22-generic x86_64
.tmp.unity_support_test.0:

ApportVersion: 2.20.1-0ubuntu2.1
Architecture: amd64
CompizPlugins: [core,composite,opengl,compiztoolbox,decor,vpswitch,snap,mousepoll,resize,place,move,wall,grid,regex,imgpng,session,gnomecompat,animation,fade,staticswitcher,workarounds,scale,expo,ezoom,dbus]
CompositorRunning: compiz
CompositorUnredirectDriverBlacklist: '(nouveau|Intel).*Mesa 8.0'
CompositorUnredirectFSW: true
CurrentDesktop: Unity
Date: Sun Jul 31 15:40:08 2016
DistUpgraded: Fresh install
DistroCodename: xenial
DistroVariant: ubuntu
DkmsStatus:
 virtualbox, 5.0.24, 4.4.0-22-generic, x86_64: installed
 virtualbox, 5.0.24, 4.4.0-31-generic, x86_64: installed
GconfCompiz:
 /apps/compiz-1/general:
   /apps/compiz-1/general/screen0:
    /apps/compiz-1/general/screen0/options:
     active_plugins = [core,composite,opengl,compiztoolbox,decor,vpswitch,snap,mousepoll,resize,place,move,wall,grid,regex,imgpng,session,gnomecompat,animation,fade,staticswitcher,workarounds,scale,expo,ezoom,dbus]
GraphicsCard:
 Advanced Micro Devices, Inc. [AMD/ATI] Seymour [Radeon HD 6400M/7400M Series] [1002:6760] (prog-if 00 [VGA controller])
   Subsystem: ASUSTeK Computer Inc. Radeon HD 7470M [1043:2002]
MachineType: ASUSTeK Computer Inc. K54HR
ProcKernelCmdLine: BOOT_IMAGE=/boot/vmlinuz-4.4.0-22-generic root=UUID=f5309739-ba9f-41fa-abaa-4b6bd1e6c9c1 ro quiet splash
SourcePackage: xorg
Symptom: display
UpgradeStatus: No upgrade log present (probably fresh install)
dmi.bios.date: 08/13/2012
dmi.bios.vendor: American Megatrends Inc.
dmi.bios.version: K54HR.206
dmi.board.asset.tag: ATN12345678901234567
dmi.board.name: K54HR
dmi.board.vendor: ASUSTeK Computer Inc.
dmi.board.version: 1.0
dmi.chassis.asset.tag: ATN12345678901234567
dmi.chassis.type: 10
dmi.chassis.vendor: ASUSTeK Computer Inc.
dmi.chassis.version: 1.0
dmi.modalias: dmi:bvnAmericanMegatrendsInc.:bvrK54HR.206:bd08/13/2012:svnASUSTeKComputerInc.:pnK54HR:pvr1.0:rvnASUSTeKComputerInc.:rnK54HR:rvr1.0:cvnASUSTeKComputerInc.:ct10:cvr1.0:
dmi.product.name: K54HR
dmi.product.version: 1.0
dmi.sys.vendor: ASUSTeK Computer Inc.
version.compiz: compiz 1:0.9.12.2+16.04.20160714-0ubuntu1
version.ia32-libs: ia32-libs N/A
version.libdrm2: libdrm2 2.4.67-1ubuntu0.16.04.1
version.libgl1-mesa-dri: libgl1-mesa-dri 11.2.0-1ubuntu2
version.libgl1-mesa-dri-experimental: libgl1-mesa-dri-experimental N/A
version.libgl1-mesa-glx: libgl1-mesa-glx 11.2.0-1ubuntu2
version.xserver-xorg-core: xserver-xorg-core 2:1.18.3-1ubuntu2.2
version.xserver-xorg-input-evdev: xserver-xorg-input-evdev 1:2.10.1-1ubuntu2
version.xserver-xorg-video-ati: xserver-xorg-video-ati 1:7.7.0-1
version.xserver-xorg-video-intel: xserver-xorg-video-intel 2:2.99.917+git20160325-1ubuntu1
version.xserver-xorg-video-nouveau: xserver-xorg-video-nouveau 1:1.0.12-1build2
xserver.bootTime: Thu Jul 28 17:32:42 2016
xserver.configfile: default
xserver.errors:

xserver.logfile: /var/log/Xorg.0.log
xserver.outputs: Output HDMI-0 LVDS VGA-0
xserver.version: 2:1.18.3-1ubuntu2.2
xserver.video_driver: radeon