I was having the same problem and did some research on-line about it. I found some sites that claimed it was a issue with the version of Android being run on the Incredible 2 and how it scans the SD card.
I don't really know what causes it, but I do know that your pics are still on the phone and you need the phone to rescan the SD card. Someone mentioned that their pics show up after a restart, which triggers the phone to rescan the SD card.
The other option I found was a free app that triggers a rescan of the SD card. You can find it in the app store. Its called SDrescan. It works really well. I would say I have to use it about once every two weeks. After you run the rescan which takes about 15 secs, everything shows up in your gallery like normal.