image field - how to overwrite existing image?
Its not there -- this functionality has been discussed on Drupal.org and ruled out by the maintainer of the filefield module before because it could cause the original file to be deleted before the node is saved. You can follow their argument here:

http://drupal.org/node/427212

Fortunately, if you do want the file to be replaced when you upload a new one with the same name there is an alternate module you can add for 6x/7x called Upload File Replace that does this and avoids the issue above:

http://drupal.org/project/upload_replace