A filter plugin might be a good way to go. Then you can prevent posts with external images, or send them for moderation. If you're using the WYSIWYG editor you can search for the HTML. Something like this:
public function filter_question( &$question, &$errors, $oldquestion )
if ( isset($question['content']) )
return 'You must upload an image instead of linking to an external one.';
Note: I don't think that regular expression is correct, the idea is to check for any src attribute that it not linked to your site. But I'm pretty sure the ^ character doesn't work like that outside square brackets. There should be another way to do the regex.