User picture on multi site / multi database setup
What I would do is:


Use hook_form_alter to add non-managed file upload to profile edit page.
Configure Drupal or webserver so that directory for these files was common, or so that all pictures would get uploaded to the "main" site's folder.
To a profile, I would add a link field. Configure it to be shared between sites, too.
In a submit handler (again, added using hook_form_alter) I would populate this field with the URL to uploaded image.