How can I automatically set default picture for categories and products?

[stevesh]

I don't, no. I've found that Zenners fluent in PHP who are here on the forums often offer code snippets to do the kind of thing you're asking for, but there's no way to predict what questions will catch their interest.

[Ajeh]

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:

  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 ...