With `Pages '09` (version 4.3 ¹):

 1. Select a picture
 2. Configure it completly as you wish
 3. Define it as the default image style:

  `Format > Advanced > Define Default Image Style`

![Define Default Image Style][1]

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¹)  I removed my installation of `Pages '11` due to its lack of functions. Its documentation isn't available on the Apple web server: [Pages manuals][2].


  [1]: https://i.sstatic.net/cQJ3k.png
  [2]: http://support.apple.com/manuals/#pages