iMessage won't send pictures on AT&T 3G...
...but sends fine when I'm connected to wifi. Have reset network settings, rebooted, disabled/re-enabled 3G, disabled/re-enabled 3G ... no improvement. iMessage acts like it's sending, then says "delivered," then shows the ⚠ and admits failure. Not an iMessage server issue, since it works fine on wifi. Must either be an IOS issue (6.1.3) or an AT&T issue.
iPhone 4, Windows 7