bug#2560: 23.0.91; Emacs silently fails to display some pictures

[Markus Triska <markus.triska@gmx.at>]

With p.png from:

   http://www.logic.at/prolog/p.png

when I do:

   $ emacs -Q p.png

then Emacs enters image mode, but instead of the actual picture I only
see a small black rectangle. If Emacs cannot handle a picture, I find
an error message and reason for failure preferable in this case. Best
of all would be to make it handle this picture correctly, if possible.

In GNU Emacs 23.0.91.3 (i386-apple-darwin8.11.1, GTK+ Version 2.12.9)
 of 2009-03-03 on mt-computer.local
Windowing system distributor `The XFree86 Project, Inc', version 11.0.40400000
Important settings:
  value of $LC_ALL: nil
  value of $LC_COLLATE: nil
  value of $LC_CTYPE: nil
  value of $LC_MESSAGES: nil
  value of $LC_MONETARY: nil
  value of $LC_NUMERIC: nil
  value of $LC_TIME: nil
  value of $LANG: en_GB.UTF-8
  value of $XMODIFIERS: nil
  locale-coding-system: nil
  default-enable-multibyte-characters: t