I have seen this issue too, there's nothing other then logging off or restarting that will fix it.
there is a terminal command but I'm not sure what it's actually killing.
Do you know what you are doing right before this happens?
Do you have your computer set to sleep or display sleep? do you often times close the lid without shutting it down?
I've been trying to pin point this issue to send to apple but if you have further information about it ( I don't know what causes it) Please let me know.
you can send feedback into apple to report this issue as well. It sounds like you can reproduce it regularly/it happens more frequently then it does for me.
It doesn't seem to matter what I'm doing. I rarely put my MBP to sleep or close the lid. My screen is set to turn off after 30 minutes I think.
I can be sitting there starring at messages on my Mac and get a pic sent to me (by someone else using an iPhone) and I'll see it on my phone but not on my Mac. After being away from my Mac for a while, I can scroll through the history and all the text messages are there but no images. If I open the transfers window in messages, all the images basically show as pending transfer and sit there spinning indefinitely.
It almost seems like some sort of firewall setting, like a blocked port or something. But it's strange that Messages BETA worked fine on Lion and now Messages on Mountain Lion has the issue. Nothing has changed in my whole network setup in months. The only change is the upgrade from Lion to Mountain Lion.
Set all those things to off, display sleep, computer sleep, don't close your lid, and see how long it takes for the issue to come back . I'm starting to suspect it's after the display sleeps. Although I've set the display to sleep after a minute to test this and it doesn't effect Messages. It just seems to happen sporadically, sometimes days, sometimes weeks for it to come up again.
I had this issue on the Beta, and still can't figure out what causes it,
I'm trying to reproduce it. Although I think it has to do with leaving your computer on and display sleep and time change.
Do you often leave your computer on over night?
can be sitting there starring at messages on my Mac and get a pic sent to me (by someone else using an iPhone) and I'll see it on my phone but not on my Mac.
Well at this point, it's already too late, whatever happened, happened.
I wasn't optomistic about the profile suggestion but I tried it anyways. As I suspected, it didn't help at all. Within an hour of swapping back to my original profile I stopped receiving pictures again.
Another fix I thought I'd try is to disable the firewall while I was experiencing the issue. That didn't help either.
I then attempted disabling my screen from sleeping and that seemed to help. It's been more than 24 hours and I seem to still be receiving pictures just fine. Both items I send and those that are sent to me seem to appear in Messages.
Later I will turn my Energy Saver settings back on and see if it reverts back to the issue. Again, the only energy settings I have set is to make my screen turn off after 20 minutes and I also have it set to my my HDD to sleep when possible. The only setting I changed in this test is the "Display Sleep". My MBP lid is almost never closed and I have "Computer Sleep" set to never as long as there is a power source connected.
So confirmed that when I turned my 'Display Sleep' back on it stopped working after a while. It wasn't after the first time it shut off, so it probably takes 60 minutes or so before it starts acting up.
I tried the PRAM reset... and it seemed to work for a while... but now it's worse than it was before. Pictures are only coming in sporadically. And now even text messages are sporadic.
I've been with the same problem for a couple of days, until finally today Messages simply stopped showing even a the window, so absolute inability to use it.
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).
This is an annoyance, but less than a whole operating system reinstall or losing the functionality.
After all of this is done you should regain the ability to receive photos, send photos and drag&drop photos.