I had exactly the same problem after the Jelly Bean 4.1.2 update - i.e. couldn't use the camera/save images or access gallery, and also had the additional problems of all my calendar entries being made read-only (as invitations - odd), as were my files/downloads - which I couldn't delete.
The answer (having learned the hard way!):
1. Connect your phone via the original USB cable to your computer and open Kies on your computer.
2. Back up all your contents/contacts/photos/videos etc. (you may have already done that before you upgraded the phone software)
3. Disconnect your phone from the computer and perform a factory reset (Settings>Personal>Backup and reset>Factory data reset
4. On the next screen at the bottom also check 'Format USB storage', then select 'Reset device'
NB: ALL your phone settings/content/everything will be wiped (but this unfortunately must be done to get the phone up and running with the Jelly Bean software).
5. When the phone restarts you will need to configure it and re-enter all settings. You will have the option to restore from e.g. samsung and/or google accounts (the google restore option may restore your apps, but I didn't select this and then had to reinstall all my apps from the Play Store (via the My Apps link) individually).
6. Reconnect your phone via USB to your computer and via Kies select the restore option (you can select what you want to restore) - unfortunately my Apps were not restored hence having to reinstall all of them one by one.
And there you go. Your phone will be up and running again. Quite like the new panorama function which works better than before and at higher resolution, amongst the other updates in the jellybean package.