Its also important to note that in cases where the sites are still there, it is most likely a faulty link or a downed server that is to blame. In the case of a faulty link, theres really nothing you can do about that unless it is fixed. A server however has whats called "bandwidth". This is a limit set by the server and when the limit of bytes has been reached, the server will be temporarily unavailable until it is restored. Then of course, theres the chance the server is down or is experiencing technical difficulties. Where this is the case, the images and/or site will be restored when the server is functioning properly again.
In any event, its better to check back with some of those links or images. Because in alot of cases its not that theres anything wrong with them, its the server. So checking back frequently is a good idea. Some servers and links are more reliable than others, so in the end it really boils down to the server or the accuracy of the link itself. In the cases of links and images it may be that they were typed in incorrectly but are still on the site in question, displaying properly.