I believe one reason should be added:
- In some countries, the ISP may be blocking access to the site where the pictures are hosted. In this case, if you enter the URL of a blocked picture in your browser, you'll probably receive a message from your ISP.
(Typically, requests for a gif or jpg will be diverted to an html page, so the browser will show a missing picture. But if the user enters the URL in the address field, then the html will be shown.)