I'm not sure if the older pictures will unencrypt ok, but if they do, one thing you can do is upload them to a picture website. I use
iTookThisOnMyPhone.com - Automatic Wireless Photo Upload
because it's free. Flickr for example has an upload limit on the free account. Those are the only two sites I know with a BB app. If you can find another, it might work as well.
It won't be a mass unencryption of the files, but with ittomp you can go into media and browse the pictures, hit the bb key and "Send to ITookThisOnMyPhone". Then while it's uploading it in the background, move to the next photo and repeat.
Not very elegant but much faster than the other methods of unencrypting such as copying the file to memory and back, sending via bluetooth, or emailing.
Later, go to the website and download the files to your PC. Make sure you set the upload location to a private album in the ittomp app, or they'll be displayed publicly until you change the status.