JPG is a bitmap file type used in game development. For more information see the Wikipedia page JPEG.
This format is often used for web games, because it can be highly compressed for faster download. After the download it is usually converted into a different in-game format to use in the game.
Warning: This is a bad format to use during while creating the source assets because it is compressed, so you lose image quality. It should only be used as a final step when publishing the game.