1. to enter the code of an image
2. displayed within the activation html mail would be nice.
3. There are notices for each mailadress (googlemail, hotmail ...) to help the user activate images in emails from your community.
Lots of advantages here, which are only possible with external images activated:
1. one is that you have a better open rate in paid newsletter advertisements. http://en.wikipedia.org/wiki/Open_rate
2. The other advantage is that you have lots of users trust your content more. May be a good trust-factor for external services.
3. Third your users get nicer mails. No bug reports about missing images in html mails any more.