I have lots of thought but not many are relevant to this question.
Silliness aside I will describe in which order Messages will Display Pics and from what sources.
Lets presume you have an AIM login.
You yourself are broadcasting your own pic and it will instantly Update to all your Buddies when your change.
It is also kept by the AIM Servers for when you are Off Line or using a method of logging in that does not use pic (web Browser access via AIM Express for example).
The reverse is true of your AIM Buddies. As they change pics then your are updates in your Buddy list. If they are Off line and you are displaying Off Line Buddies then you see the last pic AIM has for them.
IF this Buddy is in your List as Screen Name only there is no Address Card.
If you Add a First Name, Last Name to the address Card (Do a Get Info on the Buddy) then they get added to the Contacts app list (Formerly the Address Book) and it will use the Pic from Messages (or iChat if a prior Buddy)
Jabber lists (Google and Facebook IDs are Jabber IDs) send the pic once at Application Launch or if you change the Pic.
Again Buddies added with "Real Names" to the Address Book use the Messages pic.
For instance if you login to Google via a Browser it will set that pic as the current one (if you have one set) and it will override the current Messages one.
IF you do things the other way round by having a person in the Contacts list and have allocated a Pic to their Card then Messages will tend to use this Pic when you add their AIM or Jabber ID to the Card (which will then show up in Messages)
Or you link the ID/Screen Name directly to the card when adding the Buddy toi the Buddy list(s)
You will also see that when you highlight A Buddy in the List and do a "Get Info" (CMD + i keys) then the Addres Card tab lhas a box for "Always use this Pic" which can override the Address Book Pic and the pic sent from the Buddy (or server)
iMessages relies on you having a Pic in the Contact list for that person.
Of course this may be different to the ones used by the Buddy lists part of Messages.
The Pic you may see in the "Address Card" may not be the one in the Contacts App
If the Contacts Card itself is without a Pic then the person's iMessages will not have a Pic whereas their AIM or Jabber Buddy list pic will show in the "Address card" spot.
Please, if posting Logs, do not post any Log info after the line "Binary Images for iChat"
iMac 2.5Ghz 5i 2011 (Mountain Lion 10.8.2)
G4/1GhzDual MDD (Leopard 10.5.8)
MacBookPro 2Gb (Snow Leopard 10.6.8)
Mac OS X (10.6.8),
Couple of iPhones and an iPad
"Limit the Logs to the Bits above Binary Images." No, Seriously