As part of a client project we recently had the requirement of displaying an image field's default image in the node add form, before the user had uploaded a picture. This seemed simple enough to implement but thinking about it I realized this was quite a generic feature request and that such a module might prove useful for others. Rather by accident I even noticed that Drupal.org itself employs the same feature on user profile pages.
Thus, I hereby present the Image widget default image module, which does just that: If you have provided a default image for an image field and the user has not yet uploaded an image for that field, it displays that default image as a preview. Not many bells or whistle, but it works. (I can claim that, because it comes with tests. ;-)) If it does not work for you or if you have any questions please leave a comment or open an issue on Drupal.org.