If the goal is to have a specific category image display when there isn't a categories_image defined, you could customize things such as:
/includes/modules/category_row.php

using your templates and overrides ... and change the image:
Code:
  while (!$categories->EOF) {
    if (!$categories->fields['categories_image']) !$categories->fields['categories_image'] = 'pixel_trans.gif';
    $cPath_new = zen_get_path($categories->fields['categories_id']);
and change it from the clear image to one of your own ...