How can I change the naming convention of the stock camera/gallery app?
That is not currently possible on the stock camera application, however there is an open ticket with the Android project team to make such an option exposed.
http://code.google.com/p/android/issues/detail?id=10561

Meanwhile you can try some custom camera applications like "Camera Folders" which allows custom file name conventions in addition to folder groupings.

https://market.android.com/details?id=com.phasip.camerafolders&amp;feature=search_result#?t=W251bGwsMSwxLDEsImNvbS5waGFzaXAuY2FtZXJhZm9sZGVycyJd