Those are all visible to me. Try refreshing the browser window when images won't appear.
The images that people post in the topics are not hosted on the forums server. The images are stored on other host servers such as photobucket, imgur, picasa, etc. So it is possible that some images may disappear if the person who owns the images removes them from their host. It is also possible for the servers where the images are to be busy, slow or down. However, all the ones linked to all seem to be still present.