PNG, or Portable Network Graphics is a common image file format that is lossless, can contain an transparency channel, and compressed. An ideal format for distributing static assets without any loss in quality.
A turn-based game mechanic is the opposite of a real-time game mechanic. In a Turn-By-Turn mechanic (synonym), each player has to wait for the other to have played, so that they can play, and so on. This way, each player has the time to think about their strategy. TBT games rely less on action than real-time games.
