Considering that reinstalling the operating system is NOT a solution, I searched for something myself.
Pictures are controlled by xpcd, imagent and Messages, and it seems that when the problem arises is because they're using the wrong Apple server.
While this may be solved by Apple in a update (I hope so), in the meantime you can do a workaround (it worked for me on two different machines, and even days old images appeared).
In your Library, there is a folder called Preferences (that is /Users/<your_username>/Library/Preferences). There you should delete com.apple.imagent.plist com.apple.imservice.iMessage.plist and com.apple.imessage.bag.plist, with Messages CLOSED. Then reboot so Messages creates new ones. Also delete all file starting with com.apple.imagent, com.apple.imservice, com.apple.imessage and com.apple.iChat that end in .lock.
If even then it still is not working, deleting all files starting with com.apple.imagent, com.apple.imservice, com.apple.imessage and com.apple.iChat 100% for sure. But when you reboot you'll find you need to input again your login data for iMessage, FaceTime, Yahoo chat, AIM chat, Google Talk and Jabber (if you used them on Messages/iChat).