How do you include a user's default image in a Drupal 7 view that lists people?
I have just found the answer to this question via Drupal.org. You have to use a URI like this:

public://user_pictures/default.png


in your account settings.

Here, public:// stands for your public files location (often sites/default/files/). What comes after that, is the location of your image within public files. If you do this, the default image is treated like normal user images.