Ubuntu desktop wallpaper displays lower resolution than both monitor and source file

I have a photo which I'd like to set as my desktop background. It's quite a high resolution photo. I've got it set as the wallpaper in Zoom mode, so nothing's stretched but it still fills the monitor.

I have two monitors, one 1440x900, the other 1920x1200.

When I boot my computer up (I am using 14.04.1 LTS, everything up-to-date), I get a nasty low-resolution wallpaper on my larger monitor. My smaller monitor looks fine. I suspect that it is creating a small wallpaper and scaling it up for the large monitor, rather than a large wallpaper and scaling it down for the small monitor.

If I right click the desktop and choose "Change Desktop Background", then change Zoom to Centre, then change back from Centre to Zoom, it fixes the issue - until next restart!

Please see attached screenshots. View it at 1:1 / full resolution and pay particular attention to the bushes in the mid-foreground on the larger monitor, or the details on the moss up front. Looking at that spot, compare poor-res.png with high-res.png - you will see quite a clear difference!

I have also attached the full size image along with the dialogue where I'm switching from Zoom->Centre->Zoom each time I boot up.

https://www.dropbox.com/s/e9ev24f2wziyqkg/images.tar.gz

If I can tweak config files to resolve the issue or give you more information, I'm happy to do so :)

Thanks