They'll always stay encrypted. That way you can have encrypted files on there even though new ones aren't encrypted. I have pictures of friends encrypted in case I lose the phone.
To decrypt them, what you have to do is copy them over again and the device will decode them.
Several ways to do this, but basically all have to be done by the BB. One is to copy them onto the device memory then back to the card. Or you can email them, send over bluetooth connection, and maybe mms if tiny. If you do the copy to device memory then back to the card, encryption has to be disabled. The other methods don't require that, it decrypts them before sending. Have to be done one at a time.
Another way in case you have a lot is itookthisonmyphone which is one of the banner ads here. You can upload them to the website and they'll be unencrypted. Then download them to your PC and move them back to the phone. Welcome to iTookThisOnMyPhone.com - Automatic Wireless Photo Upload