I use a lot of images in my PowerPoint presentations and have a wonderful program called SnagIt installed on my PC which enables me to capture all or part of what's on my computer screen. (Of course, if you don't have a screen capture tool, you can always press the PrintScreen key to send a copy of your entire screen to the clipboard, and then edit it with Paint, for example).
Unfortunately, there's no SnagIt app for the iPad, but it is possible to capture whatever's on your iPad screen. Just press the Power (or sleep/wake) button at the top right of the iPad app and the Home button at the same time for a second or so (see image above). The screen will flash white for a moment and you will hear the camera shutter sound. The resulting screenshot can be found in your Camera Roll album in the iPad's built-in Photos app. You can use Photos' Edit function to crop, rotate or enhance your image. For more advanced photo editing work, you'll need a third-party app, such as Photogene or Snapseed.
Use the free Skitch for iPad app to add arrows, shapes and text to your images.