In a Google Docs presentation, is it possible to extract part of an inserted image and use it as a new image?
Google Docs Presentation only has a very limited set of image options ranging only to resizing, rotating, and moving. Any additional image manipulation must be done with other software. Many options are available in this area, from free to big dollars, such as Paint.NET and GIMP up to major programs such as Photoshop. Each of these editors have their own advantages and disadvantages depending on what exactly you need to do. If you only need to do basic cropping then you can use Paint on Windows or Paintbrush on Mac which both come free with your OS.