The Smuxi IRC client (version 0.8.10) sends notifications for IRC highlights with an embedded picture as blob (icon_data parameter of the spec):

Here's the output of dbus-monitor to explain what I mean:

method call sender=:1.105 -> dest=org.freedesktop.Notifications serial=45 path=/org/freedeskto
p/Notifications; interface=org.freedesktop.Notifications; member=Notify
   string "smuxi-frontend-gnome"
   uint32 0
   string ""
   string "#smuxi"
   string "<meebey> buxy: highlight"
   array [
      string "show"
      string "Afficher"
   ]
   array [
      dict entry(
         string "category"
         variant             string "im.received"
      )
      dict entry(
         string "icon_data"
         variant             struct {
               int32 256
               int32 256
               int32 1024
               boolean true
               int32 8
               int32 4
               array of bytes [
                  00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
[... here I'm snipping screens and screens of binary data ...]
                  00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
                  00 00 00 00
               ]
            }
      )
   ]
   int32 -1

When the shell receives such a notification, it just freezes 2-3 seconds before displaying it. For instance, any window/desktop change happens with 2-3 seconds lag. Strangely even my typing in gnome-terminal is blocked in that period.

One can argue that smuxi should not send that binary blob in the message itself but the spec allows it and IMO the Shell should support this better, maybe it needs to be modified to read those data asynchronously ? (I have no idea of the current implementation, I did not check)