While I know a lot of the focus for this problem is browsers, my particular problem arose when I moved from version 6.3 to version 7.0 of my firewall. I confirmed this by trying to upload one picture using IE, Firefox, Opera and Chrome, none of which worked.
Many people have automatic update set in their firewalls and may not even realize that some particular update might have caused the problem. I always set security programs to alert me to available updates so that I can trace potential problems like this, after the update I perform, when they arise.
Chuck