hook_block provide default for op

Please provide a default value for image_block() as documented in [1] and [2].
I am trying to call drupal_bootstrap() from outside of Drupal and keep getting error
Warning: Missing argument 1 for image_block()
when I try bootstrap_invoke_all('block') and image.module is enabled.

Patch attached.

[1] http://api.drupal.org/api/4.7/function/block_example_block
[2] http://api.drupal.org/api/4.7/function/hook_block