Either set your screen to 16bpp, or upgrade to at least hildon-desktop-2.2.141c-9.1
Same as on Maemo. Code: > ls ~/.backgrounds/ background-1.png background-6.png thumbnail-2.png background-2.png background-7.png thumbnail-3.png background-3.png background-8.png thumbnail-4.png background-4.png cache.info background-5.png thumbnail-1.png
> ls ~/.backgrounds/ background-1.png background-6.png thumbnail-2.png background-2.png background-7.png thumbnail-3.png background-3.png background-8.png thumbnail-4.png background-4.png cache.info background-5.png thumbnail-1.png