wallpaper with multiple monitors incorrectly displays on login

[Rowan]

If I set up a wallpaper with either the "Zoom" or "Centre" options under Appearance in System Settings with two monitors then after logging in, my laptop screen seems to display incorrectly. With zoom, the laptop screen zooms the image too much (perhaps to match the zoom for the resolution of the second screen?) and with centre, it shrunk the horizontal sides (left black bars) of the wallpaper displayed on the laptop screen in one instance, though I'm not sure of the behaviour (the image size matched the screen resolution, which is why it was weird).

This only starts happening *after* login. The wallpaper is displayed correctly in the login screen until the desktop is shown. I *think* the external monitor is unaffected, but the laptop wallpaper changes look after login.

Laptop screen resolution: 1366x768
Separate monitor resolution: 1280x1024
Wallpaper size: 1366x768 (Though this does persist with other image resolutions, e.g. 1920x1080 zooms into a tiny part of the image on the laptop screen rather than zooming out).

Ubuntu 14.04 LTS
Unity 7.2.3 (Started happening just after update to this version)
... full information has apparently been grabbed by Apport.

Sorry about the verbosity.

ProblemType: Bug
DistroRelease: Ubuntu 14.04
Package: unity 7.2.3+14.04.20140826-0ubuntu1
ProcVersionSignature: Ubuntu 3.13.0-39.66-generic 3.13.11.8
Uname: Linux 3.13.0-39-generic x86_64
.tmp.unity.support.test.0:

ApportVersion: 2.14.1-0ubuntu3.5
Architecture: amd64
CompizPlugins: [core,composite,opengl,compiztoolbox,decor,vpswitch,snap,mousepoll,resize,place,move,wall,grid,regex,imgpng,session,gnomecompat,animation,fade,unitymtgrabhandles,workarounds,scale,expo,ezoom,unityshell]
CompositorRunning: compiz
CompositorUnredirectDriverBlacklist: '(nouveau|Intel).*Mesa 8.0'
CompositorUnredirectFSW: true
CurrentDesktop: Unity
Date: Thu Nov 13 12:26:14 2014
DistUpgraded: Fresh install
DistroCodename: trusty
DistroVariant: ubuntu
GraphicsCard:
 Intel Corporation Mobile 4 Series Chipset Integrated Graphics Controller [8086:2a42] (rev 07) (prog-if 00 [VGA controller])
   Subsystem: Hewlett-Packard Company Device [103c:360b]
   Subsystem: Hewlett-Packard Company Device [103c:360b]
InstallationDate: Installed on 2014-05-11 (185 days ago)
InstallationMedia: Ubuntu 14.04 LTS "Trusty Tahr" - Release amd64 (20140417)
MachineType: Hewlett-Packard Compaq Presario CQ60 Notebook PC
ProcKernelCmdLine: BOOT_IMAGE=/boot/vmlinuz-3.13.0-39-generic root=UUID=429b2ebe-dc30-4f8f-8b96-59e07eef1089 ro quiet splash vt.handoff=7
SourcePackage: unity
UpgradeStatus: No upgrade log present (probably fresh install)
dmi.bios.date: 02/04/2009
dmi.bios.vendor: Hewlett-Packard
dmi.bios.version: F.33
dmi.board.asset.tag: Base Board Asset Tag
dmi.board.name: 3612
dmi.board.vendor: Wistron
dmi.board.version: 09.48
dmi.chassis.asset.tag: Chassis Asset Tag
dmi.chassis.type: 10
dmi.chassis.vendor: Wistron
dmi.chassis.version: Chassis Version
dmi.modalias: dmi:bvnHewlett-Packard:bvrF.33:bd02/04/2009:svnHewlett-Packard:pnCompaqPresarioCQ60NotebookPC:pvrF.33:rvnWistron:rn3612:rvr09.48:cvnWistron:ct10:cvrChassisVersion:
dmi.product.name: Compaq Presario CQ60 Notebook PC
dmi.product.version: F.33
dmi.sys.vendor: Hewlett-Packard
version.compiz: compiz 1:0.9.11.2+14.04.20140714-0ubuntu1
version.ia32-libs: ia32-libs N/A
version.libdrm2: libdrm2 2.4.52-1
version.libgl1-mesa-dri: libgl1-mesa-dri 10.1.3-0ubuntu0.1
version.libgl1-mesa-dri-experimental: libgl1-mesa-dri-experimental N/A
version.libgl1-mesa-glx: libgl1-mesa-glx 10.1.3-0ubuntu0.1
version.xserver-xorg-core: xserver-xorg-core 2:1.15.1-0ubuntu2.1
version.xserver-xorg-input-evdev: xserver-xorg-input-evdev 1:2.8.2-1ubuntu2
version.xserver-xorg-video-ati: xserver-xorg-video-ati 1:7.3.0-1ubuntu3.1
version.xserver-xorg-video-intel: xserver-xorg-video-intel 2:2.99.910-0ubuntu1.2
version.xserver-xorg-video-nouveau: xserver-xorg-video-nouveau 1:1.0.10-1ubuntu2
xserver.bootTime: Thu Nov 13 12:03:12 2014
xserver.configfile: default
xserver.errors:

xserver.logfile: /var/log/Xorg.0.log
xserver.outputs:
 product id 51584
 vendor LCD
xserver.version: 2:1.15.1-0ubuntu2.1