Complete the indicated task to unlock the corresponding picture in the Gallery menu.

Picture 30 (Last Dancer): Complete all missions (Earth Side and Bydo) then complete 10 missions again.
Picture 31 (Earth Side): Complete all missions (Earth Side and Bydo) then complete 20 missions again.
Picture 32 (Bydo Side): Complete all missions (Earth Side and Bydo) then complete 30 missions again.