When I was trying to workaround a performance bug of gnome-shell (#683829) I noticed that notification-daemon 0.7.5 seems to ignore the image-path (also tried image_path) hints. The image-path hint is defined as per DNS 1.1 and 1.2 here: http://developer.gnome.org/notification-spec/#icons-and-images-formats

The notification is shown but without an icon. When I use icon_data, that works as expected, but is deprecated and has performance issues with gnome-shell (see above).

As a workaround app_icon works as expected, but I doubt this is intended behavior.

In my case the used icon file exists, was readable and is a valid image:
meebey@redhorse:~$ test -r /usr/share/icons/hicolor/256x256/apps/smuxi-person-chat.png; echo $?
0
meebey@redhorse:~$ file /usr/share/icons/hicolor/256x256/apps/smuxi-person-chat.png
/usr/share/icons/hicolor/256x256/apps/smuxi-person-chat.png: PNG image data, 256 x 256, 8-bit/color RGBA, non-interlaced
meebey@redhorse:~$ 

Here the dbus-monitor transcript:

method call sender=:1.1195 -> dest=org.freedesktop.Notifications serial=15 path=/org/freedesktop/Notifications; interface=org.freedesktop.Notifications; member=Notify
   string "smuxi-frontend-gnome"
   uint32 0
   string ""
   string "meebey"
   string "<meebey> blubb6"
   array [
      string "show"
      string "Show"
   ]
   array [
      dict entry(
         string "category"
         variant             string "im.received"
      )
      dict entry(
         string "desktop-entry"
         variant             string "smuxi-frontend-gnome"
      )
      dict entry(
         string "image_path"
         variant             string "file:///usr/share/icons/hicolor/256x256/apps/smuxi-person-chat.png"
      )
      dict entry(
         string "sound-name"
         variant             string "message-new-instant"
      )
      dict entry(
         string "sound-file"
         variant             string "/usr/share/sounds/freedesktop/stereo/message-new-instant.oga"
      )
   ]
   int32 -1
method return sender=:1.43 -> dest=:1.1195 reply_serial=15
   uint32 129