how do i open ipod photo cache

You don't need to touch the iPod photo cache, that should only contain versions of the photos that you've synced to the iPhone, not that you've taken with it.

To copy photos taken with the iPhone or saved from emails/websites to your computer then see this page.

For photos that were originally synced from a computer you will need a third-party app such as Simple Transfer which can copy them off via your wifi network. But as photos are 'optimised' when they are synced to the iPhone, any that you then copy back to a computer may not be exactly the same as they originally were on your computer.