After starting 'emacs -Q', eval the following:

(add-hook 'image-mode-hook (lambda ()
                             (auto-revert-mode 1)))

Next perform the following steps:
1) load a PNG file (opens in image-mode buffer)
2) modify PNG file
3) notice that PNG in image-mode buffer does NOT automatically revert

In GNU Emacs 24.3.1 (i386-mingw-nt6.1.7601)
 of 2013-03-17 on MARVIN
Windowing system distributor `Microsoft Corp.', version 6.1.7601
Configured using:
 `configure --with-gcc (4.7) --cflags
 -ID:/devel/emacs/libs/libXpm-3.5.8/include
 -ID:/devel/emacs/libs/libXpm-3.5.8/src
 -ID:/devel/emacs/libs/libpng-dev_1.4.3-1/include
 -ID:/devel/emacs/libs/zlib-dev_1.2.5-2/include
 -ID:/devel/emacs/libs/giflib-4.1.4-1/include
 -ID:/devel/emacs/libs/jpeg-6b-4/include
 -ID:/devel/emacs/libs/tiff-3.8.2-1/include
 -ID:/devel/emacs/libs/gnutls-3.0.9/include
 -ID:/devel/emacs/libs/libiconv-1.13.1-1-dev/include
 -ID:/devel/emacs/libs/libxml2-2.7.8/include/libxml2'

Important settings:
  value of $LANG: ENU
  locale-coding-system: cp1252
  default enable-multibyte-characters: t

Major mode: Image[png]

Minor modes in effect:
  tooltip-mode: t
  mouse-wheel-mode: t
  tool-bar-mode: t
  menu-bar-mode: t
  file-name-shadow-mode: t
  global-font-lock-mode: t
  font-lock-mode: t
  blink-cursor-mode: t
  auto-composition-mode: t
  auto-encryption-mode: t
  auto-compression-mode: t
  line-number-mode: t
  transient-mark-mode: t

Recent input:
h e n SPC a u t o - r e v e r t SPC m o d e SPC i n
<backspace> <backspace> e n a b l e d SPC f o r SPC
i m e g a <backspace> <backspace> <backspace> a g -
<backspace> e - m o d e <C-left> <C-left> <C-left>
<C-left> <C-left> <C-left> <C-left> <C-left> o n SPC
f i l e SPC m o d i f i c a t i o n SPC <end> . <return>
S t a r t i n g SPC w i t h SPC ` e m a c s SPC - `
' : <return> <return> L a u n c h SPC e m a c s . SPC
SPC L o a d SPC a C-x k <return> y e s <return> C-x
C-g C-x o C-x o k q C-x b s c r <tab> <return> C-x
C-f ~ <backspace> . e m <tab> i n <tab> <return> <C-end>
<up> <up> <up> <up> <up> <up> <up> <up> <up> <up> <up>
<up> <up> <up> <up> <up> <up> <up> <up> <up> <up> <up>
<up> <up> <up> <up> <up> <up> <up> <up> <up> <up> <up>
<up> <up> <up> <up> <up> <up> <up> <up> <up> <up> <up>
<up> <up> <up> <up> <down> <down> <down> <down> <up>
<up> <delete> <delete> <delete> <down> <delete> <delete>
<tab> <up> <tab> <down> <tab> <up> <home> <S-down>
<S-down> <escape> w C-x b <return> C-y <left> C-x C-e
C-x b <tab> m a t <tab> <return> <return> C-x C-f C-g
C-x o C-x C-f S e <tab> u <return> <C-home> <down>
<down> <end> <left> <left> <left> <left> <left> <left>
<left> <left> a C-x C-s <down-mouse-1> <mouse-1> <down-mouse-1>
<mouse-1> <down-mouse-1> <mouse-1> <escape> x r e p
o <tab> r <tab> <return>

Recent messages:
Checking for load-path shadows...done
Loading vc-git...done
Mark set [3 times]
((lambda nil (auto-revert-mode 1)))
Making completion list...
Quit
Mark set
Saving file c:/Users/agooch/Projects/match-tdd/SeqDiag-0-User-initiates-match.pu...
Wrote c:/Users/agooch/Projects/match-tdd/SeqDiag-0-User-initiates-match.pu
Making completion list...

Load-path shadows:
None found.

Features:
(vc-git help-mode shadow sort gnus-util mail-extr emacsbug message
format-spec rfc822 mml easymenu mml-sec mm-decode mm-bodies mm-encode
mail-parse rfc2231 mailabbrev gmm-utils mailheader sendmail rfc2047
rfc2045 ietf-drums mm-util mail-prsvr mail-utils image-mode dired
time-date tooltip ediff-hook vc-hooks lisp-float-type mwheel dos-w32
ls-lisp w32-common-fns disp-table w32-win w32-vars tool-bar dnd fontset
image regexp-opt fringe tabulated-list newcomment lisp-mode register
page menu-bar rfn-eshadow timer select scroll-bar mouse jit-lock
font-lock syntax facemenu font-core frame cham georgian utf-8-lang
misc-lang vietnamese tibetan thai tai-viet lao korean japanese hebrew
greek romanian slovak czech european ethiopic indian cyrillic chinese
case-table epa-hook jka-cmpr-hook help simple abbrev minibuffer loaddefs
button faces cus-face macroexp files text-properties overlay sha1 md5
base64 format env code-pages mule custom widget hashtable-print-readable
backquote make-network-process w32 multi-tty emacs)