I just had the exact same thing happen to several 8320 users this weekend. We were all in a group and one of them sent a pic to the group and all the 8320's went into this boot loop.
You need to remove the card, and plug it into a pc. You may need to get an adapter that converts it from a microSD to an SD card or one that converts it to a thumb drive. Then delete those pictures and, the thumbs.xxx file, if one exists.
If you don't have an adapter, then try putting the card back in the device and plug it into the computer while it's booting up. If the device is set for mass storage mode then the card will not be recognized by the device because it will be recognized by the pc. Then go to My Computer and delete the pics.
If it's not in mass storage mode when it boots up, watch for a prompt on the device immediately after it boots up. See if you can get it into mass storage before it reboots itself. If it's not in mass storage and doesn't prompt you, then you may need to quickly navigate to Options >Media Card and enable mass storage before it reboots itself again.
If you can't get that to work, then you'll need to go find and adapter. Most places carry thumb drive type holders that you can plug the card into to make it a microSD card reader.