this bug has been fixed in Ubuntu 14.10
$ file /usr/share/backgrounds/warty-final-ubuntu.png /usr/share/backgrounds/warty-final-ubuntu.png: PNG image data, 2560 x 1600, 8-bit/color RGB, non-interlaced
this bug has been fixed in Ubuntu 14.10
$ file /usr/share/ backgrounds/ warty-final- ubuntu. png backgrounds/ warty-final- ubuntu. png: PNG image data, 2560 x 1600, 8-bit/color RGB, non-interlaced
/usr/share/