update docs and error messages
use png extension of files, since they are
don't create image files by default
use input filename for image files
Change-Id: I3e9c31a1de02976432d3f39cf82b5403055d620a
this was self-consistent on the device, but the host tool
treats lack of padding in a different way.
Bug:10778984
Change-Id: I47f653a98f23ff0ce54218d100e0c8e2f92b67fa